Trifluridine

Trifluridine is a potent nucleoside analog antiviral medication used topically for the treatment of ocular infections caused by feline herpesvirus type 1 (FHV-1) in cats. Originally developed and approved for human herpes simplex virus keratitis, trifluridine has been adopted in veterinary medicine for off-label treatment of similar herpetic eye conditions in cats. The medication is available as a 1% ophthalmic solution under the brand name Viroptic and is considered one of the most effective topical antivirals against FHV-1 based on in vitro studies, though its use in cats is often limited by local irritation and the need for frequent application.

Trifluridine works through a mechanism that inhibits viral DNA synthesis. Structurally, it is a fluorinated pyrimidine nucleoside analog of thymidine, one of the building blocks of DNA. After entering infected cells, trifluridine undergoes phosphorylation by both cellular and viral enzymes to its active triphosphate form. This active compound competes with natural thymidine for incorporation into viral DNA, and when incorporated, it causes errors and premature termination of the DNA chain, effectively stopping viral replication. Unlike some antivirals that depend primarily on viral thymidine kinase for activation, trifluridine's phosphorylation pathway involves cellular enzymes as well, which may contribute to both its efficacy and its potential for host cell toxicity.

The medication is administered as eye drops directly to the affected eye or eyes, typically requiring frequent application five to six times daily to maintain effective drug concentrations at the ocular surface. This frequent dosing schedule reflects trifluridine's virostatic mechanism, meaning it suppresses viral replication rather than killing the virus outright, requiring continuous presence at effective concentrations to prevent viral recovery between doses. Treatment duration varies depending on the condition being treated and the cat's response, but prolonged use beyond two to three weeks is generally avoided due to potential corneal toxicity.

While trifluridine demonstrates superior corneal penetration and potent antiviral activity compared to some alternatives, its use in cats is frequently complicated by ocular irritation. Many cats exhibit discomfort, including stinging and increased tear production, upon application of the medication. This tolerability issue, combined with the demanding dosing frequency, often leads veterinarians to consider alternative antivirals for many feline patients. Nevertheless, trifluridine remains an important option in the treatment of FHV-1 ocular disease, particularly in cases where other antivirals have proven ineffective or are unavailable. Veterinary guidance is essential for appropriate use and monitoring throughout treatment.

The primary indication for trifluridine in cats is the treatment of ocular disease caused by feline herpesvirus type 1 (FHV-1), the most common cause of infectious conjunctivitis and keratitis in domestic cats. FHV-1 infection is widespread in the feline population, with serological studies indicating exposure rates exceeding 90% in some populations. While many cats harbor latent infections without clinical signs, recurrent ocular disease can develop when the virus reactivates during periods of stress or immunosuppression. Trifluridine provides a topical antiviral option for managing active ocular disease when treatment is indicated.

Herpetic keratitis, particularly dendritic corneal ulcers, is a classic indication for topical antiviral therapy including trifluridine. Dendritic ulcers are characterized by branching, tree-like patterns of corneal epithelial erosion that are highly suggestive of FHV-1 infection. These ulcers result from active viral replication in corneal epithelial cells and can be painful, causing blepharospasm, ocular discharge, and reluctance to open the affected eye. In vitro studies have demonstrated that trifluridine is among the most potent antiviral agents against FHV-1, with efficacy exceeding that of idoxuridine, vidarabine, and acyclovir in laboratory testing. This potency makes it a consideration for cases requiring aggressive antiviral therapy.

Herpetic conjunctivitis, with or without concurrent keratitis, is another indication for trifluridine use. Cats with FHV-1 conjunctivitis typically present with conjunctival hyperemia, chemosis, and ocular discharge that may be serous initially and become mucopurulent with secondary bacterial infection. While many cases of herpes conjunctivitis are self-limiting and resolve without specific antiviral therapy, more severe or persistent cases may benefit from topical antiviral treatment. Trifluridine's superior corneal penetration compared to some alternatives may make it particularly useful when both corneal and conjunctival tissues are affected.

Veterinarians may select trifluridine over alternative topical antivirals in specific clinical scenarios. When idoxuridine, often the first-line topical antiviral due to better tolerability, has proven ineffective, trifluridine may be tried as a second-line option given its different mechanism may overcome resistance patterns. In geographic regions where certain antivirals are unavailable, trifluridine may be the accessible option for treating herpetic ocular disease. The medication may also be selected when rapid viral suppression is desired and the cat is expected to tolerate the more intensive treatment regimen.

It is important to note that trifluridine is not appropriate for all ocular conditions in cats and has specific limitations. The medication is only effective against herpesvirus and does not treat bacterial, fungal, or parasitic infections. Accurate diagnosis of FHV-1 involvement is important before initiating therapy, though the characteristic clinical presentation often supports empirical treatment. Trifluridine is too toxic for systemic administration and should only be applied topically to the eye. The medication does not address latent virus harbored in trigeminal ganglia and therefore cannot prevent future recurrences of ocular disease, though it effectively suppresses active viral replication in affected ocular tissues during treatment.

Trifluridine dosing in cats follows protocols established for human herpetic keratitis, adapted for veterinary use with consideration of feline-specific tolerability concerns. The standard formulation is a 1% ophthalmic solution, with one drop applied to the affected eye or eyes. Precise adherence to the prescribed dosing regimen is essential for treatment success, as the virostatic mechanism of trifluridine requires consistent drug presence to prevent viral replication recovery. Cat owners should receive thorough instruction on administration technique and scheduling from their veterinarian.

The typical dosing frequency for trifluridine in cats with herpetic ocular disease is one drop applied to the affected eye five to six times daily, approximately every four hours during waking hours. This intensive dosing schedule reflects the drug's mechanism and the need to maintain effective concentrations at the corneal surface. Some protocols recommend an initial intensive phase with even more frequent dosing, followed by gradual reduction as clinical improvement occurs. The demanding dosing schedule often presents challenges for compliance, particularly for working pet owners, and veterinarians may need to discuss practical scheduling strategies to maximize adherence.

Treatment duration with trifluridine is typically limited to two to three weeks, with most protocols recommending a minimum of seven days of treatment and avoidance of prolonged therapy beyond 21 days. Shorter courses may be attempted for mild infections with rapid clinical response, while more severe or resistant cases may require the full treatment duration. Extended treatment beyond three weeks increases the risk of corneal epithelial toxicity and is generally avoided unless specifically indicated and closely monitored. Veterinarians should reassess cats that have not shown adequate improvement within the expected timeframe.

Proper administration technique helps maximize drug delivery while minimizing stress for the cat. The medication bottle should be stored according to label directions, typically refrigerated between 2°C and 8°C. Before administration, the bottle can be briefly warmed in the hands to reduce the shock of cold drops on the eye. The cat should be gently restrained, and the lower eyelid pulled down slightly to create a pocket for the medication. One drop is instilled into this pocket, avoiding direct contact between the dropper tip and the eye or surrounding tissues to prevent contamination. After administration, the eyelid can be gently closed to help distribute the medication across the ocular surface.

If a dose of trifluridine is missed, it should be administered as soon as remembered, and the regular dosing schedule resumed. There is no need to double the next dose, as this would not provide therapeutic benefit and could increase local irritation. Given the frequency of dosing required, occasional missed doses are common and generally do not compromise treatment outcome if most doses are administered as scheduled. However, chronic poor compliance significantly reduces treatment effectiveness, and caregivers experiencing difficulty maintaining the dosing schedule should discuss alternatives with their veterinarian.

Completion of the prescribed treatment course is important to ensure adequate viral suppression, even if clinical signs appear to have resolved before the scheduled end of treatment. Premature discontinuation may allow residual viral activity to recover, potentially leading to recurrence of clinical disease. However, unlike antibiotics where completion of a full course is necessary to prevent resistance, antiviral therapy duration is somewhat more flexible and can be adjusted based on clinical response. The veterinarian should provide guidance on when discontinuation is appropriate based on the individual cat's progress.

Trifluridine has a well-characterized side effect profile in ophthalmic use, with local ocular effects being the primary concern. Understanding these potential effects helps cat owners monitor their pet appropriately during treatment and recognize when veterinary consultation is needed. The side effect profile is one of the main factors limiting trifluridine's use in feline patients, as tolerability is often poorer in cats compared to some alternative antiviral options.

The most common and significant side effect of trifluridine in cats is ocular irritation upon application. Many cats exhibit marked discomfort including blepharospasm (squinting), increased lacrimation (tearing), and attempts to paw at or rub the treated eye. This stinging reaction, which is also reported in human patients, appears to be particularly pronounced in cats and may make them resistant to subsequent doses. The irritation typically occurs immediately upon instillation and may persist for several minutes. While not dangerous in itself, this discomfort can significantly impact compliance with the demanding dosing schedule and may lead some owners to discontinue treatment prematurely.

Mild conjunctival hyperemia, or redness of the tissues around the eye, is a common finding during trifluridine treatment. This may be difficult to distinguish from the hyperemia caused by the underlying herpes infection itself, though worsening redness during treatment or redness appearing in a previously unaffected eye may indicate a reaction to the medication. Mild lid edema and occasional periocular irritation may also occur. These effects are generally self-limiting and resolve after treatment is discontinued but should be reported to the veterinarian if they are severe or concerning.

Prolonged use of trifluridine can lead to more significant corneal toxicity. Because trifluridine inhibits DNA synthesis in both viral and host cells, extended treatment can damage the rapidly dividing cells of the corneal epithelium. Signs of corneal toxicity include punctate epithelial keratopathy (small spots of corneal damage visible with fluorescein staining), delayed epithelial healing, and in severe cases, persistent epithelial defects. This risk is the primary reason treatment duration is typically limited to three weeks or less. Cats receiving extended treatment courses require close monitoring for signs of corneal toxicity.

Serious adverse effects beyond local ocular irritation are uncommon with topical trifluridine use. Systemic absorption from ophthalmic administration is minimal, and systemic toxicity is not expected with appropriate topical use. However, trifluridine should never be administered orally or by injection, as systemic exposure would cause significant toxicity including bone marrow suppression. Allergic reactions to trifluridine are rare but possible, with signs potentially including severe swelling, marked conjunctival changes, or skin reactions around the eye. Any suspected allergic reaction should prompt discontinuation and veterinary evaluation.

Cat owners should contact their veterinarian if their cat experiences severe or worsening ocular irritation, apparent increase in eye discharge or pain, development of new symptoms in the treated or opposite eye, or any other concerning changes during treatment. The veterinarian can assess whether the symptoms represent expected medication effects, progression of the underlying disease, or a complication requiring treatment modification.

While trifluridine is generally considered safe for topical ophthalmic use in cats when appropriately prescribed, several contraindications and precautions apply. Understanding these limitations helps ensure the medication is used safely and that alternative treatments are selected when trifluridine is not appropriate. Veterinarians evaluate each patient individually to determine if topical antiviral therapy is indicated and, if so, which agent is most suitable given the cat's specific circumstances.

Cats with known hypersensitivity to trifluridine or any component of the ophthalmic formulation should not receive the medication. While true allergic reactions to trifluridine are uncommon, any cat that has previously experienced an allergic reaction, characterized by severe periocular swelling, urticaria, or signs of systemic allergic response, should not be retreated. Cats showing simple irritation from previous use are not necessarily allergic but may be candidates for alternative antivirals with better tolerability profiles.

Trifluridine should not be used for extended periods beyond the typically recommended two to three week maximum duration due to risk of corneal epithelial toxicity. Cats requiring longer-term antiviral therapy for chronic or frequently recurrent herpetic disease may be better served by alternative medications such as oral famciclovir, which has a favorable safety profile for extended use, or other topical agents with less potential for corneal toxicity. When continued topical antiviral therapy is necessary beyond typical treatment duration, close ophthalmologic monitoring is essential.

The medication is contraindicated for systemic administration and should only be applied topically to the eye. Oral or injectable administration of trifluridine would cause serious systemic toxicity including bone marrow suppression. Cat owners should be clearly instructed that the medication is for eye use only and should never be administered orally or by any other route. If accidental ingestion of the medication occurs, veterinary advice should be sought promptly, though the small volumes contained in ophthalmic bottles generally limit the risk of significant toxicity from such accidents.

Pregnancy and lactation considerations apply to trifluridine use. While topical ophthalmic absorption is minimal, trifluridine has mutagenic potential and its safety during feline pregnancy has not been established. Pregnant queens with herpetic ocular disease should be treated with consideration of potential risks, and the veterinarian may recommend alternative agents with better established safety profiles during pregnancy. Similarly, nursing queens treated with trifluridine may theoretically excrete small amounts in milk, though the clinical significance is likely minimal given the small systemic absorption from topical use. These decisions should be made on a case-by-case basis considering the severity of the ocular disease and available alternatives.

Drug interactions with topically applied trifluridine are generally limited compared to systemically administered medications, owing to the minimal systemic absorption achieved with ophthalmic use. However, several considerations apply regarding concurrent ophthalmic medications and the timing of administration. Cat owners should inform their veterinarian of all medications their cat receives, including other eye medications, to allow appropriate management of potential interactions.

When multiple topical ophthalmic medications are prescribed concurrently, appropriate spacing between applications is important to allow adequate absorption of each medication and prevent dilution or washout effects. A general guideline is to wait at least five to ten minutes between different eye medications. If trifluridine is prescribed alongside other drops or ointments for concurrent conditions such as bacterial infection or inflammation, the veterinarian should provide specific guidance on administration sequence and timing. Typically, aqueous solutions like trifluridine are administered before ointment-based medications, which can create a barrier to absorption of subsequent drops.

The concurrent use of topical corticosteroids with trifluridine requires careful consideration. Corticosteroids can enhance viral replication by suppressing local immune responses and are generally contraindicated during active epithelial herpetic disease. However, certain FHV-1 related conditions such as stromal keratitis and eosinophilic keratitis may benefit from concurrent anti-inflammatory therapy once active epithelial viral disease is controlled. The decision to use topical steroids concurrently with antiviral therapy should be made by a veterinarian, preferably one with expertise in ophthalmology, who can assess the specific clinical situation and balance benefits against risks.

Other topical antiviral medications are generally not used concurrently with trifluridine, as combining antivirals is unlikely to provide additional benefit and could increase local toxicity. If a cat has failed to respond adequately to one topical antiviral, the veterinarian may recommend switching to an alternative agent rather than adding a second topical antiviral. Systemic antivirals such as oral famciclovir may be used in combination with topical therapy in severe cases, as the different routes of administration can provide complementary coverage. This combination approach may be particularly useful for cats with both corneal and deeper ocular involvement.

Concurrent systemic medications are unlikely to interact significantly with topically applied trifluridine given the minimal systemic absorption achieved with ophthalmic use. However, cats receiving immunosuppressive medications for other conditions may have altered responses to both the underlying herpetic infection and treatment. These cats may require more aggressive or prolonged antiviral therapy and closer monitoring for treatment response. The veterinarian should be aware of all systemic medications to appropriately plan antiviral treatment.

L-lysine supplementation, often recommended for cats with FHV-1 infections, does not interact with trifluridine and may be continued during topical antiviral treatment. However, the efficacy of lysine supplementation for FHV-1 has been questioned by recent research, and its role as an adjunct to antiviral therapy remains uncertain. Veterinarians may or may not recommend concurrent lysine supplementation based on their assessment of the available evidence.

Several important precautions and warnings apply to the use of trifluridine in cats. Awareness of these considerations helps ensure safe and effective use while minimizing the risk of complications. Both veterinary professionals prescribing the medication and cat owners administering it should understand these precautions to optimize treatment outcomes.

Proper diagnosis of FHV-1 related ocular disease should ideally precede initiation of trifluridine therapy. While the clinical presentation of herpes keratitis, particularly dendritic ulcers, may be highly suggestive, other causes of corneal ulceration and conjunctivitis exist and would not respond to antiviral therapy. Misdiagnosis could lead to inappropriate treatment, progression of the actual underlying condition, and potential toxicity from unnecessary medication use. Diagnostic testing for FHV-1, including PCR of ocular swabs, may be helpful in confirming the diagnosis, particularly in cases with atypical presentations.

Trifluridine should be used with caution in cats with pre-existing corneal disease or compromised corneal integrity beyond that caused by the active herpes infection. Cats with thin or fragile corneas, previous corneal surgery, or other conditions affecting corneal health may be at increased risk for toxicity from trifluridine's effects on corneal epithelial cells. Ophthalmologic evaluation before and during treatment is particularly important in these patients to monitor corneal health and detect early signs of toxicity.

Cat owners should receive thorough instruction on proper administration technique to maximize treatment efficacy while minimizing contamination risks. The dropper tip should never touch the eye, eyelids, or any other surface to prevent bacterial contamination of the solution. Contaminated medication could introduce bacteria to an already compromised ocular surface, potentially causing secondary bacterial infection. Hands should be washed before and after administering eye medication, and the bottle should be stored according to label directions when not in use.

Monitoring during trifluridine treatment should include regular assessment of treatment response and observation for adverse effects. Cat owners should watch for signs of excessive irritation, apparent worsening of the eye condition, or development of new symptoms. Veterinary rechecks may be scheduled during treatment, particularly for severe cases or cats receiving extended therapy. Any concerning changes should prompt earlier evaluation. Complete ophthalmologic examination including fluorescein staining can help assess treatment response and detect corneal toxicity.

The medication should be kept out of reach of children and other pets, as accidental ingestion could potentially cause harm. While the small volumes in ophthalmic containers limit serious toxicity risk, ingestion should be avoided and medical or veterinary advice sought if it occurs. The medication is for veterinary use only as prescribed and should not be shared between animals or used to treat other conditions without veterinary guidance.

Cat owners should understand that topical antiviral therapy treats active viral disease but does not eliminate latent virus or prevent future recurrences. Many cats with FHV-1 experience periodic reactivation of ocular disease throughout their lives. Strategies for minimizing stress-induced recurrences, maintaining overall health, and recognizing early signs of recurrent disease can help manage this chronic condition long-term. The veterinarian can provide guidance on comprehensive management beyond acute treatment episodes.

Proper storage and handling of trifluridine ophthalmic solution is essential for maintaining medication stability, sterility, and effectiveness throughout the treatment course. The medication has specific storage requirements that must be followed to preserve its antiviral activity. Cat owners should receive clear instructions from their veterinarian or pharmacist on proper handling procedures.

Trifluridine ophthalmic solution should be stored in the refrigerator at temperatures between 2°C and 8°C (36°F to 46°F). The medication should not be frozen, as freezing may damage the formulation and reduce effectiveness. Within the refrigerator, the bottle should be stored upright in its original container, protected from light. Keeping the medication in a consistent location helps ensure it is readily accessible for the frequent dosing schedule while maintaining proper storage conditions.

Before each administration, the refrigerated medication can be briefly warmed to reduce the discomfort of cold drops on the eye. Holding the bottle in closed hands for a minute or two is sufficient to bring the solution closer to body temperature without compromising stability. The medication should not be heated in warm water or any other heat source, as excessive heating could degrade the active ingredient. Room temperature storage for brief periods during administration is acceptable, but the bottle should be returned to the refrigerator promptly after use.

Proper handling during administration helps maintain sterility of the ophthalmic solution. The dropper tip should never contact the eye, eyelid, skin, or any other surface, as this can introduce bacteria into the solution. If accidental contamination of the dropper tip occurs, the veterinarian should be consulted about whether the medication should continue to be used or replaced. The bottle cap should be replaced securely after each use. Hands should be washed before handling the medication and before and after administering drops to the cat.

Trifluridine should be discarded and replaced if the solution becomes cloudy, changes color, or contains visible particles. These changes may indicate contamination or degradation of the medication. The expiration date on the bottle should be observed, and medication should not be used beyond this date. Additionally, many ophthalmologists recommend discarding open bottles of ophthalmic medication after a specified period, typically 28 to 30 days, regardless of remaining volume, to minimize contamination risk. The veterinarian can advise on appropriate discard intervals for the specific product dispensed.

Disposal of unused or expired trifluridine should follow appropriate guidelines for pharmaceutical waste. The medication should not be poured down drains or discarded in regular household trash where it could contaminate water supplies or be accessed by children or animals. Many pharmacies and veterinary offices accept unused medications for proper disposal. Local pharmaceutical take-back programs may also accept veterinary medications. The veterinarian or pharmacist can provide guidance on appropriate disposal methods in accordance with local regulations.

Feline herpesvirus type 1 affects cats of all breeds, and trifluridine can be used for treating herpetic ocular disease regardless of breed. No specific breed-related contraindications to trifluridine have been identified, though certain breed characteristics may influence treatment considerations or expectations. Understanding these factors helps optimize care for individual patients while recognizing that the fundamental approach to herpetic eye disease treatment is similar across breeds.

Some purebred cats may have higher prevalence of FHV-1 related ocular disease compared to mixed-breed cats, potentially reflecting both increased exposure in cattery and breeding environments and possible genetic factors affecting immune responses or ocular anatomy. Breeds sometimes associated with increased risk or severity of herpetic eye disease include Persians, Himalayans, and other brachycephalic breeds. However, this increased incidence reflects susceptibility to FHV-1 ocular disease rather than any difference in response to trifluridine treatment itself.

Brachycephalic breeds with prominent eyes and shallow orbits may present unique considerations for ophthalmic treatment administration. The prominent eye position may make medication administration somewhat easier in terms of access but may also increase the risk of corneal exposure and drying between doses. These breeds often have pre-existing tear film abnormalities that can complicate ocular surface disease and healing. Concurrent management of tear film deficiency with artificial tears or other supportive therapies may be beneficial alongside antiviral treatment in affected brachycephalic cats.

Breeds predisposed to corneal sequestrum formation, particularly Persians, Himalayans, and related breeds, deserve special mention in the context of FHV-1 treatment. Corneal sequestra are areas of corneal necrosis that may develop in association with chronic herpetic keratitis, among other causes. While trifluridine treats active viral infection, it does not directly address sequestrum formation, which may require surgical intervention. Cats of predisposed breeds with herpetic keratitis should be monitored for sequestrum development, and comprehensive management should address both the viral infection and any complications.

Size and temperament variations across breeds may influence the practical aspects of treatment administration. Larger cats are not necessarily more difficult to treat but may require different handling techniques for medication administration. Cats with more fractious temperaments, regardless of breed, may be challenging to treat with the frequent dosing schedule required for trifluridine. In such cases, the veterinarian may need to consider whether the cat's temperament allows for adequate compliance with the intensive treatment regimen or whether alternative antivirals with less frequent dosing might be more practical.

Young kittens of any breed are particularly susceptible to severe primary FHV-1 infection, which can cause significant ocular damage including symblepharon formation if not treated promptly. Trifluridine can be used in kittens, with dosing based on the same one drop per eye guideline used in adults. The smaller eye size of kittens means that one drop provides adequate coverage. Special care should be taken during restraint and administration to avoid injuring the delicate tissues of very young patients.

Several medications related to trifluridine are used in the treatment of feline herpesvirus type 1 ocular disease. Understanding the relationships between these agents helps contextualize treatment options and provides information about alternatives when trifluridine is not suitable or available. The choice among available antivirals depends on factors including efficacy, tolerability, availability, cost, and the specific clinical situation.

Idoxuridine is a closely related nucleoside analog that is often considered the first-line topical antiviral for feline herpetic keratitis due to its generally better tolerability compared to trifluridine. Like trifluridine, idoxuridine is a thymidine analog that inhibits viral DNA synthesis when incorporated into the growing DNA strand. While in vitro studies suggest idoxuridine is somewhat less potent than trifluridine against FHV-1, its better tolerability in cats often makes it the preferred initial choice. Idoxuridine is not commercially available in ready-to-use form in many regions and typically must be obtained from compounding pharmacies as a 0.1% solution or 0.5% ointment.

Vidarabine is another topical antiviral option that works through a different mechanism than trifluridine and idoxuridine, affecting a different step in viral DNA synthesis. This mechanistic difference may allow vidarabine to be effective in cases resistant to thymidine analogs. Available as a 3% ophthalmic ointment, vidarabine is often reported to be well tolerated by cats and may be particularly useful for patients who cannot tolerate solution formulations. Like idoxuridine, vidarabine typically requires compounding as it is not widely commercially available.

Famciclovir represents a different approach to treating FHV-1 infections, providing systemic rather than topical antiviral therapy. This oral medication is converted to its active form penciclovir, which has demonstrated efficacy against FHV-1. Famciclovir offers advantages of once or twice daily dosing and ability to reach not only the ocular surface but also deeper ocular structures and other sites of viral activity. It is often used for severe or recurrent FHV-1 disease and may be combined with topical antivirals for comprehensive treatment.

Cidofovir is a topical antiviral that has shown efficacy against FHV-1 in experimental studies. Its primary advantage is that it requires only twice daily application due to the long tissue half-lives of its metabolites, compared to the five to six times daily dosing required for trifluridine. This more practical dosing schedule can significantly improve compliance. However, cidofovir is not commercially available as an ophthalmic preparation and must be compounded, and some concerns exist about potential nasolacrimal drainage complications with long-term use.

Cat owners should never substitute human antivirals for veterinary-prescribed medications without veterinary guidance. Notably, valacyclovir, which is effective for human herpes infections, is highly toxic to cats and should never be administered. The safe use of antiviral medications in cats requires veterinary oversight to select appropriate agents, doses, and monitoring protocols for the individual patient.