Remdesivir (GS-5734)

Remdesivir, also known by its developmental code GS-5734, is a groundbreaking nucleotide analog antiviral medication that has revolutionized the treatment of feline infectious peritonitis (FIP) in cats. Originally developed by Gilead Sciences for treating human coronavirus infections including SARS-CoV-2, remdesivir has found remarkable success in veterinary medicine as an off-label treatment for this previously fatal feline disease. The medication represents one of the most significant advances in feline medicine in recent decades, transforming FIP from a uniformly fatal diagnosis to a highly treatable condition with cure rates reported between 80 and 96 percent in treated cats.

Remdesivir functions as a prodrug of GS-441524, meaning it is converted into its active metabolite form after administration. Once inside infected cells, remdesivir undergoes metabolic conversion to GS-441524, which is then phosphorylated to its active triphosphate form (GS-443902). This active compound mimics adenosine, a building block of viral RNA, and becomes incorporated into the growing viral RNA strand during replication. When incorporated, it causes premature termination of viral RNA synthesis, effectively halting the coronavirus from reproducing within the cat's body. This mechanism makes remdesivir highly effective against the feline coronavirus mutations responsible for FIP.

Remdesivir is available in multiple formulations for feline use, including injectable solutions for intravenous and subcutaneous administration, as well as compounded oral formulations. The injectable form is typically used during the initial treatment phase, particularly in severely ill cats requiring immediate intervention. Oral formulations, when available, offer a more convenient option for the extended treatment course typically required to achieve remission. Treatment protocols generally involve daily administration for a minimum of 84 days, though duration may be extended based on the cat's response and the form of FIP being treated.

The safety profile of remdesivir in cats has been demonstrated through extensive clinical use worldwide. While injection site reactions and mild gastrointestinal effects may occur, the medication is generally well-tolerated when administered according to veterinary protocols. The development and availability of remdesivir for FIP treatment represents a collaborative effort between veterinary researchers, particularly those at UC Davis, and has given hope to thousands of cat owners facing this devastating diagnosis. Veterinary supervision is essential throughout the treatment course to monitor for efficacy, adjust dosing as needed, and manage any adverse effects that may arise.

The primary and most significant indication for remdesivir in cats is the treatment of feline infectious peritonitis (FIP), a progressive and previously fatal disease caused by virulent mutations of feline coronavirus. FIP occurs when the common feline enteric coronavirus mutates within an individual cat, gaining the ability to infect macrophages and spread systemically throughout the body. Before the advent of effective antiviral treatments like remdesivir, FIP was considered universally fatal, with affected cats typically surviving only days to weeks after diagnosis. Remdesivir has fundamentally changed this prognosis, offering survival rates of 80 to 96 percent in properly treated cats.

Remdesivir is effective against both major clinical presentations of FIP. The effusive or wet form of FIP is characterized by accumulation of protein-rich fluid in body cavities, particularly the abdomen and chest. Studies have demonstrated that oral remdesivir at appropriate doses is highly effective for effusive FIP, with many cats showing dramatic improvement within the first few days of treatment. The non-effusive or dry form of FIP presents with granulomatous lesions in various organs and may be more challenging to treat, sometimes requiring higher doses or extended treatment duration. Clinical trials have shown that while survival rates in non-effusive cases may be somewhat lower than in effusive cases, the majority of cats with both forms can achieve remission with appropriate therapy.

Beyond the two primary presentations, remdesivir is also used for treating FIP with neurological involvement and ocular FIP. Neurological FIP occurs when the virus affects the central nervous system, causing signs such as ataxia, seizures, behavioral changes, and vestibular dysfunction. While historically considered the most difficult form to treat, studies have shown that higher doses of remdesivir can achieve therapeutic concentrations in cerebrospinal fluid sufficient to combat the virus. Ocular FIP presents with uveitis and other inflammatory eye conditions, and research has demonstrated that antiviral treatment effectively resolves these ocular manifestations in the majority of cases.

Veterinarians may choose remdesivir over other antiviral options based on several factors. In regions where remdesivir is legally available through compounding pharmacies or as the human-approved product used off-label, it may be preferred due to its established efficacy and relatively well-characterized safety profile. Remdesivir may also be selected for initial parenteral treatment in severely ill cats who cannot tolerate oral medication, with transition to oral GS-441524 once the patient stabilizes. The choice between remdesivir and GS-441524 may also depend on local availability, cost considerations, and veterinary experience with different treatment protocols.

Patient selection for remdesivir treatment typically includes cats with a confirmed or highly suspected diagnosis of FIP based on clinical signs, laboratory findings, and imaging. Early intervention is associated with better outcomes, making prompt diagnosis and treatment initiation crucial. Cats of any age can be treated, though young cats and kittens are most commonly affected by FIP. The treatment is considered regardless of FIP form, though veterinarians may adjust protocols based on disease severity and organ involvement.

The dosing of remdesivir for feline infectious peritonitis requires careful veterinary oversight, as protocols have evolved based on clinical experience and research findings. It is essential that cat owners work closely with their veterinarian to determine the appropriate dosing regimen for their individual cat, as factors such as disease severity, form of FIP, body weight, and response to treatment all influence the optimal approach. The following information provides general guidance but should not replace professional veterinary advice for any specific patient.

For parenteral administration, remdesivir is typically initiated at doses ranging from 10 to 15 mg/kg given intravenously or subcutaneously once daily. Some protocols recommend an induction phase using intravenous administration for the first several doses, particularly in severely ill or dehydrated cats, followed by transition to subcutaneous injections. The intravenous route ensures rapid drug delivery and allows for simultaneous fluid therapy if needed. Subcutaneous administration is effective and more practical for outpatient treatment, though injection site reactions may occur. When giving subcutaneous injections, rotating injection sites across the dorsum helps minimize local irritation.

Oral remdesivir formulations, when available through compounding pharmacies, are typically dosed at approximately 25 to 30 mg/kg once daily. Studies have demonstrated that oral remdesivir at 30 mg/kg daily is effective for effusive FIP, though cats with non-effusive disease may require higher doses or more frequent administration. The oral route offers significant advantages for long-term treatment, eliminating the need for daily injections and reducing stress for both cats and their caregivers. Oral formulations should be administered consistently at the same time each day to maintain therapeutic drug levels.

The standard treatment duration for FIP with remdesivir is 84 days, or 12 weeks, of continuous daily therapy. This extended treatment course is necessary to achieve sustained viral suppression and allow the cat's immune system to clear infected cells. Some cats may require longer treatment periods, particularly those with neurological or non-effusive FIP, or cats that show incomplete response to initial therapy. Veterinarians monitor patients throughout treatment to assess response and determine if the standard duration is sufficient or if extended therapy is warranted.

Administration of remdesivir should be performed as directed by the prescribing veterinarian. For injectable forms, proper aseptic technique is essential, and caregivers administering subcutaneous injections at home should receive thorough instruction on injection technique, site selection, and signs of complications. The medication is typically stored in the refrigerator and warmed to room temperature before injection to minimize discomfort. Oral formulations may be given with or without food, depending on the specific product and individual cat tolerance.

If a dose of remdesivir is missed, it should be given as soon as remembered, and the regular dosing schedule resumed. Doses should not be doubled to make up for missed doses, as this could increase the risk of adverse effects without providing additional therapeutic benefit. Consistent daily administration is crucial for treatment success, and caregivers should establish routines to help ensure doses are not forgotten. If multiple doses are missed or if there are concerns about adherence, the veterinarian should be contacted to discuss options and potentially reassess the treatment plan.

Completion of the full treatment course is critical for achieving lasting remission. Discontinuing treatment early, even if the cat appears fully recovered, risks relapse of FIP. Studies have shown that cats that complete the full 84-day course have significantly better long-term outcomes than those with incomplete treatment. After completing treatment, cats should be monitored during a post-treatment observation period to ensure sustained remission, with veterinary rechecks recommended at specific intervals to assess for any signs of disease recurrence.

Remdesivir has demonstrated a favorable safety profile in cats when used for FIP treatment, with most cats tolerating the medication well throughout the extended treatment course. Clinical studies and extensive field experience have characterized the side effect profile, allowing veterinarians and cat owners to anticipate and manage potential adverse effects effectively. Understanding what to expect during treatment helps caregivers provide optimal supportive care and recognize when veterinary attention may be needed.

The most commonly reported side effect with injectable remdesivir is injection site discomfort or irritation, particularly with subcutaneous administration. Cats may vocalize or react during injection, and some develop mild skin reactions at injection sites including temporary swelling, redness, or hair loss. These local reactions are generally mild and self-limiting, resolving as injection sites are rotated and given time to heal. Using proper injection technique, warming the medication to room temperature before administration, and rotating sites systematically helps minimize these effects. In most cases, injection site reactions do not require treatment modification.

Some cats may experience mild depression or lethargy following remdesivir administration, particularly after intravenous infusion. This effect is typically transient, lasting only a few hours after each dose, and does not indicate a serious adverse reaction. Gastrointestinal effects including decreased appetite, nausea, or occasional vomiting may occur in some cats, particularly during the initial treatment period. These effects are usually mild and often resolve as treatment continues. Administering oral formulations with food may help reduce gastrointestinal upset in susceptible individuals.

More significant side effects are uncommon but require prompt veterinary attention. Signs of allergic reaction, though rare, would include facial swelling, hives, difficulty breathing, or sudden collapse and constitute a medical emergency. Any dramatic behavioral changes, persistent vomiting, severe lethargy, or refusal to eat for more than 24 hours should be reported to the veterinarian promptly. Some cats may show transient elevations in liver enzymes during treatment, which is typically monitored through periodic blood work. Significant or progressive liver enzyme elevations may require treatment modification.

Unlike the related compound valacyclovir, which causes severe and potentially fatal toxicity in cats, remdesivir has not been associated with bone marrow suppression, hepatic necrosis, or renal failure when used at appropriate doses. This safety distinction is crucial and underscores the importance of using only veterinary-approved antiviral protocols rather than substituting other human antiviral medications. The metabolic pathway of remdesivir to GS-441524 appears to be well-tolerated by the feline system, contributing to its favorable safety profile compared to other antiviral options.

Long-term side effects from remdesivir treatment in cats have not been extensively documented, as the medication's use in veterinary medicine is relatively recent. However, cats that have completed treatment and achieved remission have generally done well in follow-up, without apparent lasting adverse effects from therapy. Ongoing monitoring after treatment completion allows detection of any delayed effects and confirms sustained remission. Cat owners should maintain communication with their veterinarian throughout and after treatment to report any concerning changes and ensure optimal long-term outcomes.

While remdesivir has proven to be a life-saving medication for cats with FIP, certain conditions and circumstances may contraindicate its use or require careful consideration before initiating treatment. Understanding these contraindications helps ensure that remdesivir is used safely and appropriately, maximizing benefit while minimizing risk to feline patients. Veterinarians evaluate each cat individually to determine if remdesivir treatment is appropriate given the patient's complete health status.

Cats with known hypersensitivity or previous allergic reaction to remdesivir or any component of its formulation should not receive the medication. While allergic reactions to remdesivir are rare in cats, any cat that has experienced an adverse reaction during previous treatment should be carefully evaluated before re-exposure. Signs of allergic reaction may include facial swelling, hives, difficulty breathing, or anaphylaxis, and any cat experiencing such symptoms should receive immediate emergency care and should not receive additional doses without thorough veterinary assessment.

Severe hepatic impairment may complicate remdesivir use, as the drug undergoes metabolic processing in the liver. Cats with pre-existing significant liver disease should be carefully evaluated, and baseline liver function tests should be performed before starting treatment. While remdesivir has not been shown to cause hepatotoxicity in cats at therapeutic doses, the presence of underlying liver dysfunction may affect drug metabolism and potentially increase susceptibility to adverse effects. Veterinarians may adjust dosing or increase monitoring frequency in cats with hepatic concerns.

Similarly, cats with severe renal impairment require careful consideration, although remdesivir has not demonstrated the nephrotoxicity in cats that has been observed with related compounds like valacyclovir. Given the high prevalence of chronic kidney disease in older cats, veterinarians should assess renal function before and during treatment. Cats with significant pre-existing kidney disease may require modified protocols or enhanced monitoring to ensure safe treatment. Adequate hydration is particularly important in cats with any degree of renal compromise.

The use of remdesivir during pregnancy and lactation in cats has not been extensively studied, and potential effects on fetal development or nursing kittens are unknown. While cats diagnosed with FIP during pregnancy face a grave prognosis without treatment, the decision to treat must weigh the potential risks to developing kittens against the certain fatal outcome of untreated FIP. Breeding cats receiving remdesivir should not be bred during treatment. Queens nursing kittens should be carefully monitored, and veterinarians may recommend weaning kittens before initiating treatment if feasible given the clinical situation. These decisions require careful discussion between veterinarians and cat owners to determine the best course of action for each individual situation.

Understanding potential drug interactions with remdesivir is essential for safe and effective FIP treatment in cats. Cat owners should inform their veterinarian of all medications, supplements, and over-the-counter products their cat receives to allow assessment of possible interactions and appropriate management. While remdesivir's interaction profile in cats is not as extensively characterized as in humans, several important considerations apply based on its pharmacology and clinical experience.

Remdesivir may interact with other medications that undergo hepatic metabolism or affect liver enzyme systems. Drugs that induce or inhibit cytochrome P450 enzymes could theoretically affect remdesivir metabolism, though the clinical significance in cats is not well established. When cats require concurrent medications for other conditions, veterinarians should review all drugs for potential interactions and may recommend staggered dosing, enhanced monitoring, or selection of alternative agents when appropriate. Particular attention should be paid when combining remdesivir with other hepatically metabolized drugs in cats with pre-existing liver concerns.

Concurrent use of other antiviral medications with remdesivir requires veterinary oversight. While some treatment protocols utilize remdesivir followed by transition to oral GS-441524, simultaneous use of multiple antiviral agents may not provide additional benefit and could potentially increase toxicity risk. Cats should not receive remdesivir concurrently with other nucleoside or nucleotide analogs unless specifically directed by a veterinarian as part of an established treatment protocol. The related compound valacyclovir should never be administered to cats due to its severe toxicity, regardless of what other treatments the cat may be receiving.

Nonsteroidal anti-inflammatory drugs and corticosteroids are sometimes used as supportive therapy in cats with FIP, and their interaction with remdesivir deserves consideration. While no direct pharmacological interaction has been identified, corticosteroids may theoretically modulate immune responses relevant to FIP recovery. Some treatment protocols have historically included low-dose prednisolone during the initial treatment phase to reduce inflammation, though current evidence suggests remdesivir monotherapy is sufficient in most cases. NSAIDs for fever management should be used cautiously, particularly in cats with any concern for renal function, and the veterinarian should be aware of all anti-inflammatory use.

Supplements and herbal products may interact with remdesivir treatment, though specific interactions in cats are poorly documented. Immune-modulating supplements, including those containing echinacea, astragalus, or other immunostimulant herbs, should be discussed with the veterinarian before use during FIP treatment. L-lysine, commonly recommended for feline herpesvirus infections, does not have a known interaction with remdesivir but is not effective against FIP coronavirus. Cat owners should disclose all supplements to ensure no potentially problematic combinations occur.

During remdesivir treatment, veterinarians may monitor for signs of interaction including unexpected changes in the cat's condition, altered response to treatment, or emergence of new symptoms. Regular veterinary rechecks allow assessment of treatment progress and provide opportunity to identify any concerning patterns that might suggest drug interactions or adverse effects requiring management.

Several important precautions and warnings apply to the use of remdesivir for FIP treatment in cats. Both veterinary professionals and cat owners should be aware of these considerations to ensure safe and effective therapy. Adherence to established protocols and maintenance of regular veterinary communication throughout the treatment course helps optimize outcomes while minimizing risks.

Accurate diagnosis of FIP before initiating remdesivir treatment is essential, as the medication is specifically indicated for this condition and the extended treatment course represents a significant commitment of time and resources. FIP diagnosis can be challenging since no single definitive test exists, and diagnosis typically relies on a combination of clinical signs, laboratory findings, effusion analysis when present, and imaging studies. Veterinarians may use specific biomarkers, protein electrophoresis, and PCR testing to support the diagnosis. Treatment should ideally be initiated only after a high-confidence diagnosis to avoid unnecessary treatment of cats without FIP.

Regular monitoring throughout the treatment course is crucial for ensuring appropriate response and detecting any problems early. Veterinary rechecks are typically recommended at specified intervals during the 84-day treatment course, often at weeks one, two, four, eight, and twelve, with additional visits as needed based on the cat's progress. Monitoring parameters include physical examination, body weight, temperature, assessment of effusion resolution if present, complete blood count, serum biochemistry, and globulin concentrations. Cats showing inadequate response may require dose adjustment or extended treatment duration.

Handling precautions apply to both injectable and oral formulations of remdesivir. Caregivers administering the medication should wear gloves and wash hands thoroughly after handling. Pregnant women should avoid handling remdesivir or wear protective equipment if handling is necessary. Injectable solutions should be stored according to product specifications, typically refrigerated, and checked for particulates or discoloration before use. Any medication that appears abnormal should not be administered. Unused medication should be disposed of according to veterinary guidance rather than discarded in household trash.

In multi-cat households, special considerations apply during FIP treatment. While FIP itself results from mutation of coronavirus within an individual cat rather than transmission of the virulent form between cats, other cats in the household may be infected with the common feline enteric coronavirus. There is no evidence that treating one cat poses risks to other household cats, but general hygiene practices should be maintained. The stress of treatment and veterinary visits should be minimized for the affected cat, and providing quiet recovery space separate from household activity may benefit the patient.

Cat owners should be prepared for the significant commitment required for successful FIP treatment with remdesivir. The 84-day treatment course requires consistent daily medication administration, regular veterinary visits, and considerable financial investment. Partial treatment is not recommended as it may lead to treatment failure and potential development of viral resistance. Before initiating treatment, veterinarians typically discuss the full scope of the treatment commitment to ensure cat owners can provide the complete therapy course necessary for the best chance of successful outcome.

Post-treatment monitoring is recommended to confirm sustained remission and detect any signs of relapse early. Cats completing treatment should be monitored at home for recurrence of clinical signs and should return for veterinary recheck examinations as recommended, typically at two weeks and twelve weeks after treatment completion, and potentially beyond. Early detection of relapse allows prompt intervention with retreatment protocols that can still achieve successful outcomes in most cases.

Proper storage and handling of remdesivir is essential for maintaining medication stability and ensuring treatment effectiveness. Both injectable and oral formulations have specific requirements that must be followed to preserve the drug's therapeutic activity throughout the extended FIP treatment course. Cat owners receiving medication for home administration should receive thorough instruction from their veterinarian or pharmacist on appropriate storage and handling procedures.

Injectable remdesivir solutions typically require refrigeration at temperatures between 2°C and 8°C (36°F to 46°F). The medication should be stored in its original container, protected from light, and kept in a consistent location within the refrigerator away from the freezer compartment. Before administration, the refrigerated solution should be allowed to warm to room temperature, which helps reduce injection discomfort for the cat. Warming can be achieved by removing the vial from refrigeration approximately 30 minutes before the scheduled dose or by briefly holding the syringe in warm hands, but the medication should never be heated in a microwave or placed in hot water. Once drawn into a syringe, the medication should be administered promptly.

Compounded oral formulations of remdesivir may have different storage requirements depending on the specific formulation prepared by the compounding pharmacy. Some oral preparations require refrigeration while others may be stable at room temperature. The compounding pharmacy label should specify exact storage conditions, and these instructions should be followed precisely. Oral formulations should be stored in their original containers with tight-fitting lids, protected from light and moisture. Any specific handling instructions, such as shaking before use for liquid formulations, should be noted and followed consistently.

Proper handling procedures protect both the medication and the handler. Caregivers should wash hands before and after handling remdesivir and consider wearing disposable gloves, particularly when handling injectable formulations or if skin contact with the medication is likely. Any spilled medication should be cleaned up promptly using appropriate absorbent materials, and contaminated materials should be disposed of safely. Eye protection may be advisable when drawing up injectable doses to prevent accidental ocular exposure. Pregnant women, nursing mothers, and immunocompromised individuals should take extra precautions or avoid handling the medication if possible.

Medication disposal should follow veterinary guidance and local regulations for pharmaceutical waste. Unused or expired remdesivir should not be discarded in household trash or flushed down drains, as this can contribute to environmental contamination. Many veterinary offices and pharmacies accept medication returns for proper disposal. Some communities have pharmaceutical take-back programs that accept veterinary medications. When such programs are not available, the veterinarian can advise on appropriate disposal methods that comply with local requirements while minimizing environmental impact.

Feline infectious peritonitis occurs in cats of all breeds, and remdesivir treatment has been successfully used across the full spectrum of feline breeds and mixed-breed cats. While no specific breed-related contraindications to remdesivir have been identified, understanding breed-related factors that may influence FIP occurrence and treatment can help optimize care for individual patients. Veterinarians consider the cat's breed alongside other patient factors when developing treatment protocols.

Certain purebred cat populations may have higher incidence of FIP compared to mixed-breed cats, potentially related to genetic factors affecting immune responses or the crowded breeding and shelter environments where coronavirus exposure and transmission are more likely. Breeds sometimes cited as having increased FIP risk include Abyssinians, Bengals, Birmans, British Shorthairs, Himalayans, Ragdolls, and Devon Rex, among others. However, this increased incidence reflects susceptibility to developing FIP rather than any difference in response to remdesivir treatment. Cats of any breed with FIP can be treated with remdesivir using standard protocols.

Large-breed cats require appropriate dose calculation based on accurate body weight to ensure therapeutic drug levels. Breeds such as Maine Coons, Ragdolls, Norwegian Forest Cats, and British Shorthairs may exceed average cat weights considerably, and doses must be calculated correctly to avoid under-treatment. Conversely, smaller breeds and individual cats at the lower end of normal weight ranges need precise dosing to avoid excessive drug exposure. Weight-based dosing with regular weight monitoring throughout treatment ensures appropriate drug delivery regardless of breed or size.

Breed-specific health conditions may influence treatment management even though they do not contraindicate remdesivir use. Persian and related breeds prone to polycystic kidney disease should have renal function assessed before and during treatment. Breeds predisposed to hypertrophic cardiomyopathy, including Maine Coons, Ragdolls, and British Shorthairs, should receive cardiac evaluation as part of their overall health assessment, particularly since FIP can occasionally cause pericardial effusion or myocardial involvement. These considerations reflect general feline breed health management rather than specific remdesivir concerns.

Young cats and kittens of any breed are most commonly affected by FIP and respond well to remdesivir treatment when diagnosed and treated promptly. Pediatric patients require careful dose calculation based on current weight, with dose adjustments as the kitten grows during the treatment course. Kittens may have less mature metabolic pathways than adult cats, though this has not been shown to significantly affect remdesivir safety or efficacy. Senior cats developing FIP, though less common, can also be successfully treated, with age-appropriate health monitoring throughout the treatment course. The overall prognosis with remdesivir treatment is favorable across age groups and breeds when treatment is initiated promptly and completed fully.

Several medications are related to remdesivir either through shared mechanisms of action, use for similar conditions, or as alternative options in FIP treatment. Understanding these related medications helps contextualize remdesivir within the broader landscape of feline antiviral therapy and provides information about options that may be considered in various clinical scenarios.

GS-441524 is the most closely related medication to remdesivir, as it is the active metabolite into which remdesivir is converted in the body. GS-441524 has been extensively used for FIP treatment worldwide, often through unofficial sources before becoming more widely accessible through veterinary channels. The two medications have demonstrated equivalent efficacy in clinical studies, with both achieving high cure rates in cats with effusive and non-effusive FIP. The choice between remdesivir and GS-441524 may depend on availability, cost, formulation preferences, and veterinary experience in a given region. Some protocols utilize remdesivir for initial parenteral treatment followed by oral GS-441524 for maintenance therapy.

Molnupiravir is another antiviral that has been investigated for FIP treatment and represents a different class of antivirals with a distinct mechanism of action. While less extensively studied than remdesivir or GS-441524 for feline use, molnupiravir has shown some promise in preliminary investigations. Veterinarians may consider alternative antivirals when first-line options are unavailable or ineffective, though evidence for alternatives remains less robust. Any change to alternative antiviral agents should be made only under veterinary supervision with appropriate monitoring.

Historically, before effective antiviral treatments were available, FIP was managed with supportive care and immunomodulatory approaches including polyprenyl immunostimulant and feline interferon omega. While these treatments sometimes provided temporary improvement or extended survival in some cats, they did not achieve the cure rates seen with modern antiviral therapy. These immunomodulatory treatments may still have a role as adjunctive therapy in certain cases, though they are not substitutes for effective antiviral treatment with remdesivir or GS-441524.

It is critically important to note that certain human antiviral medications are dangerous for cats and should never be substituted for remdesivir. Valacyclovir causes potentially fatal hepatic necrosis, renal toxicity, and bone marrow suppression in cats and is absolutely contraindicated. Acyclovir has poor efficacy against feline coronavirus and can cause toxicity when given systemically to cats. Cat owners should never administer human antiviral medications to their cats without veterinary guidance, as the consequences can be fatal. Only veterinary-approved treatment protocols should be followed for FIP management.