Altrenogest (Regu-Mate) - mares

Altrenogest, marketed primarily under the brand name Regu-Mate, is a synthetic progestogen specifically approved for use in mares to suppress estrus-related behaviors and maintain pregnancy. This medication represents one of the most significant pharmacological advances in mare management, providing a safe and effective means of controlling the behavioral and physiological changes associated with the estrous cycle. Altrenogest has become indispensable in performance mare management, breeding programs, and for addressing behavioral issues related to hormonal fluctuations in female horses. The medication's ability to suppress heat behaviors while maintaining athletic performance has made it a cornerstone of competitive mare management across numerous equine disciplines.

The mechanism of action of altrenogest involves mimicking the effects of natural progesterone, the hormone that predominates during the luteal phase of the mare's estrous cycle and throughout pregnancy. By providing exogenous progestogen activity, altrenogest suppresses the release of gonadotropin-releasing hormone (GnRH) from the hypothalamus, which in turn reduces follicle-stimulating hormone (FSH) and luteinizing hormone (LH) secretion from the pituitary gland. This hormonal suppression prevents follicular development and ovulation, effectively maintaining the mare in a state similar to diestrus or early pregnancy. The behavioral effects follow from this hormonal modulation, as estrus-related behaviors such as squatting, urinating, tail raising, and distraction from work are minimized when estrogen influence is reduced.

Altrenogest is available as an oral solution formulated specifically for equine use, with the standard concentration being 0.22% (2.2 mg/mL). The medication is administered daily by mouth, typically using the calibrated dosing syringe provided with the product. Administration involves depositing the liquid directly onto the mare's tongue or mixing with a small amount of feed. The oral solution formulation allows for flexible dosing based on body weight and provides consistent drug delivery when administered correctly. Daily administration without interruption is essential for maintaining estrus suppression, as missed doses can result in return of cycling behavior within a few days.

The safety profile of altrenogest in mares is well-established through decades of clinical use, and the medication is generally well-tolerated when used according to label directions. However, human safety considerations are paramount, as altrenogest can be absorbed through the skin and has potent hormonal effects. Pregnant women, women who may become pregnant, and women of childbearing age should not handle this product without appropriate protective equipment. The critical importance of handler safety makes altrenogest unique among equine medications in requiring specific precautions beyond typical pharmaceutical handling. Veterinary guidance ensures appropriate use for the mare while emphasizing safety protocols for all individuals who may handle the medication.

The primary FDA-approved indication for altrenogest in mares is suppression of estrus to allow for more effective management during training and competition. Many mares exhibit behavioral changes during their estrous cycle that can significantly impact performance and trainability. Signs of estrus in mares include frequent urination, posturing with raised tail, vulvar winking, reduced attention and responsiveness to handler cues, and sometimes irritability or aggression toward other horses. These behaviors can interfere with training sessions, compromise safety during handling, and negatively affect competitive performance. Altrenogest effectively eliminates or substantially reduces these estrus-related behaviors, allowing mares to work consistently regardless of their natural cycle timing.

Beyond estrus suppression for performance purposes, altrenogest is widely used for managing what is colloquially termed "moody mare syndrome" or behavioral issues attributed to hormonal fluctuations. Some mares exhibit dramatic mood swings, irritability, sensitivity, or difficult behavior that corresponds with their estrous cycle. While not all difficult mare behavior is hormonally mediated, mares whose behavioral issues clearly track with cycling often respond well to altrenogest therapy. The medication allows these mares to maintain a more even temperament, improving their quality of life and making them safer and more pleasant to handle. Behavioral response to altrenogest can help confirm whether symptoms are truly estrus-related or have other underlying causes.

Altrenogest plays a critical role in equine breeding programs for pregnancy maintenance in mares with a history of early embryonic loss or inadequate progesterone production. Mares who have experienced pregnancy loss before the placenta takes over progesterone production (typically around days 120-150 of gestation) may benefit from altrenogest supplementation during the critical early pregnancy period. The medication provides exogenous progesterone support until the fetoplacental unit can sustain the pregnancy independently. This application has helped many mares successfully carry pregnancies to term who might otherwise have experienced early pregnancy failure.

Additional clinical applications for altrenogest include synchronization of estrus in groups of mares for breeding management purposes. When multiple mares need to be bred within a specific timeframe, altrenogest can be administered to suppress cycling, then withdrawn to allow synchronized return to estrus. This technique is valuable in breeding operations where veterinary resources must be efficiently allocated or when coordinating breeding with limited stallion availability. Mares being transported for breeding or moving between breeding facilities may receive altrenogest to prevent unwanted behavioral issues during transit. The medication also sees use in managing transitional mares during the spring breeding season to help establish regular cycles.

When veterinarians select altrenogest for mare management, several factors influence the decision. The medication is chosen when estrus-related behaviors interfere with performance, training, or handling; when pregnancy support is needed in at-risk mares; or when estrus synchronization serves breeding management goals. Altrenogest offers advantages of oral administration, predictable effects, and extensive clinical experience supporting its use. The primary limitation is the requirement for daily administration without interruption, which demands consistent owner compliance. Cost considerations are meaningful, as long-term altrenogest therapy represents a significant ongoing expense, though most owners consider this worthwhile for mares with substantial behavioral improvement.

Dosing of altrenogest in mares follows well-established protocols developed through extensive clinical use and research. The standard label dose is 0.044 mg/kg body weight administered orally once daily. For the commonly used 0.22% (2.2 mg/mL) solution, this translates to 1 mL of solution per 50 kg (110 lbs) of body weight daily. A typical 1,000-pound (approximately 450 kg) mare would receive approximately 9 mL daily. The medication comes with a calibrated syringe to facilitate accurate dosing based on body weight. Veterinary determination of the exact dose accounts for individual mare factors, though most mares respond well to standard dosing. Accurate weight estimation is important, as underdosing may result in inadequate estrus suppression while excessive dosing increases cost without therapeutic benefit.

For estrus suppression in performance mares, the typical protocol involves daily administration for as long as suppression is desired, often throughout the competition season or training period. Some practitioners recommend beginning administration several days before expected estrus onset when possible. Response to altrenogest typically begins within a few days of initiating therapy, with full behavioral effects usually established within one week. Consistent daily dosing is essential, as missed doses can result in return of estrus behaviors within two to four days. There is no loading dose requirement, and maintenance at the standard dose provides reliable ongoing suppression in most mares.

Pregnancy supplementation protocols may differ from behavioral management applications. Mares receiving altrenogest for pregnancy maintenance typically begin therapy when pregnancy is confirmed or when early pregnancy support is deemed necessary based on history. Treatment continues until the fetoplacental unit can independently support the pregnancy, generally around days 120-150 of gestation, though some practitioners continue longer in mares with significant pregnancy loss history. The decision to discontinue pregnancy supplementation should be made in consultation with the veterinarian, often following assessment of fetal viability and placental development. Gradual tapering rather than abrupt discontinuation may be recommended in some cases.

Administration of altrenogest requires attention to both effectiveness and safety. The oral solution should be deposited directly onto the back of the mare's tongue using the calibrated syringe, ensuring complete swallowing of the dose. Alternatively, the medication can be mixed with a small amount of palatable feed and consumed completely. Administration should occur at approximately the same time each day to maintain consistent blood levels. The handler must wear protective gloves when administering altrenogest, as the drug is readily absorbed through skin. Pregnant women, women who may become pregnant, and women of childbearing age should ideally not handle the medication at all. All handlers should avoid skin contact, wash hands thoroughly after any potential exposure, and be aware of the medication's hormonal activity.

If a dose of altrenogest is missed, it should be given as soon as possible, and the regular daily schedule should be resumed. Missing a single dose may not immediately compromise estrus suppression, but multiple missed doses will likely result in return of cycling behavior within days. Owners should contact their veterinarian if doses are missed and estrus signs begin to return. Doubling doses to compensate for missed administration is not necessary and not recommended. For mares on long-term therapy, establishing a consistent daily routine for administration helps ensure doses are not forgotten.

Treatment duration with altrenogest varies based on the indication. Performance mares may receive the medication throughout their competitive season, which could span several months to year-round depending on the discipline. Pregnancy supplementation continues until placental progesterone production is sufficient, typically around five months of gestation. When discontinuing altrenogest in non-pregnant mares, withdrawal results in return of normal cycling within several days to two weeks, depending on the time of year and individual factors. Gradual tapering is generally not required for behavioral management applications, though veterinary guidance should be followed for pregnancy supplementation discontinuation.

Altrenogest is generally well-tolerated in mares when administered at recommended doses, and most mares experience minimal adverse effects from the medication itself. Decades of clinical use have established the safety profile for equine patients, with serious adverse effects being uncommon. However, owners should be aware of potential effects and monitor their mares during treatment. The most significant safety concerns with altrenogest relate to human handler exposure rather than effects on the treated mare, though both aspects warrant attention.

The most commonly observed effect in mares receiving altrenogest is prolonged interval to return of normal estrous cycling after discontinuation. While this is an expected consequence of the medication's mechanism rather than a true side effect, owners should understand that fertility will be temporarily suppressed and normal cycling may take days to weeks to resume after stopping treatment. Some mares may show slight changes in temperament beyond the intended estrus suppression, occasionally appearing somewhat more docile or less reactive than their baseline personality. Changes in appetite or weight are occasionally reported but are typically minor and may be difficult to attribute directly to medication versus other management factors.

Some mares may experience clitoral enlargement with prolonged altrenogest use, reflecting the androgenic activity present in this synthetic progestogen. This effect is generally reversible upon discontinuation but may be concerning to owners unfamiliar with this possibility. Changes in coat quality or texture have been occasionally reported with long-term use. Injection site reactions are not applicable since altrenogest is administered orally, but gastrointestinal effects such as loose manure or mild digestive upset occur occasionally in sensitive individuals. These effects are typically self-limiting and rarely necessitate discontinuation of therapy.

More significant effects requiring veterinary attention would include signs of unexpected pregnancy complications in mares receiving altrenogest for pregnancy support, such as vaginal discharge, colic, or other concerning signs. Any mare showing signs of a systemic reaction after altrenogest administration should be evaluated. Behavioral changes opposite to the expected effect, such as increased irritability or aggressiveness, warrant reassessment of the diagnosis and treatment approach. If clitoral enlargement becomes marked or causes apparent discomfort, veterinary consultation is advisable. Long-term use effects should be periodically assessed during ongoing therapy.

The most critical safety considerations with altrenogest involve human handler exposure. The drug is readily absorbed through skin contact and has potent progestogenic and androgenic effects. Women of childbearing age face particular risks, as the medication could potentially affect menstrual cycles or, in pregnant women, cause fetal harm. Pregnant women should not handle altrenogest under any circumstances. All handlers must wear protective gloves, avoid skin contact, and wash thoroughly after any potential exposure. Spilled medication should be cleaned carefully with appropriate protection. These handling precautions distinguish altrenogest from most other equine medications and require emphasis with all individuals involved in the mare's care.

Altrenogest should not be used in mares with known hypersensitivity to the medication or any component of the formulation. While allergic reactions to altrenogest are rare, mares with documented previous adverse reactions should not receive the drug. Known sensitivity to other progestogens may also warrant caution, though cross-reactivity is not well-characterized. Disclosure of any previous adverse drug reactions to the prescribing veterinarian helps ensure appropriate medication selection and allows for consideration of alternative approaches to estrus management if needed.

Altrenogest is specifically contraindicated in stallions and geldings, as the medication is formulated and approved only for use in mares. Administration to male horses could cause hormonal disruption, potential reproductive effects in intact males, and is not supported by safety or efficacy data. The drug should not be confused with or substituted for other hormonal preparations that might be used in male horses. Similarly, altrenogest formulated for equine use should not be administered to other species without specific veterinary direction and appropriate dose adjustment for species-specific differences in drug handling.

Use in pregnant mares requires careful consideration and clear therapeutic purpose. While altrenogest is appropriately used for pregnancy maintenance in mares with documented need, unnecessary administration during pregnancy could theoretically mask signs of pregnancy failure or create hormonal complications. Mares receiving altrenogest for estrus suppression who become unexpectedly pregnant present a complex situation requiring veterinary guidance. The medication should not be administered to mares intended for breeding in the immediate future unless specifically managing return to estrus timing. Young, reproductively immature fillies do not require estrus suppression and should not receive altrenogest unless specifically indicated for behavioral management under veterinary direction.

Other considerations affecting altrenogest use include hepatic disease, as the medication undergoes liver metabolism and could potentially accumulate in horses with significant liver dysfunction. Mares with active reproductive tract infections should have these conditions addressed before initiating hormone therapy. Mares with hormone-dependent neoplasia, though rare, may be inappropriate candidates for progestogen therapy. Competition mares must observe appropriate regulations, as altrenogest may be prohibited or restricted under certain governing body rules, particularly at higher levels of competition or in specific countries. Current FEI and USEF regulations permit altrenogest use in mares under certain conditions, but rules vary and may change, requiring verification with the relevant authority.

Drug interactions with altrenogest in horses are not extensively documented, but several potential interactions warrant consideration based on the medication's hormonal mechanism and metabolic pathway. The most significant interactions involve other hormonal preparations, where combining altrenogest with additional progestogens, estrogens, or GnRH-affecting medications could produce unpredictable hormonal effects. Prostaglandin F2-alpha preparations, such as dinoprost (Lutalyse) or cloprostenol (Estrumate), are specifically used to counteract progesterone effects and induce estrus in mares. Concurrent administration of prostaglandins would counteract altrenogest's effects, though sequential use is intentional in some breeding synchronization protocols. Veterinary coordination is essential when multiple hormonal agents are part of a mare's reproductive management.

Medications affecting hepatic metabolism could theoretically influence altrenogest processing, as the drug undergoes liver metabolism. Enzyme-inducing drugs might reduce altrenogest efficacy by accelerating its metabolism, while enzyme inhibitors could potentially increase drug levels. While specific interaction data is limited in horses, awareness of concurrent medications that significantly affect liver enzyme activity is prudent. Chronic non-steroidal anti-inflammatory drug use does not appear to significantly interact with altrenogest, and the medications are commonly used concurrently in performance horses without reported problems.

Interactions with sedatives, anesthetics, and other commonly used equine medications have not been specifically documented, and altrenogest does not appear to affect the response to routine veterinary treatments. Mares receiving altrenogest can undergo normal sedation, anesthesia, and surgical procedures without specific dose adjustments for concurrent medications. Vaccination schedules are not affected by altrenogest therapy. However, any mare undergoing veterinary treatment should have her complete medication list disclosed to the treating veterinarian to allow for informed clinical decisions.

Competition drug testing considerations represent an important practical interaction for performance mares. Altrenogest is currently permitted for use in mares under FEI and USEF regulations, though this status has evolved over time and could change. Rules may differ in other countries or under other governing bodies, and some disciplines or competitions may have specific restrictions. The interaction between altrenogest use and drug testing protocols requires awareness of current regulations and proper documentation. Some competitions may require declaration of altrenogest use or presentation of veterinary documentation. Staying current with regulatory requirements helps ensure compliance while maintaining mare management protocols.

Human safety precautions are paramount when handling altrenogest and deserve primary emphasis in any discussion of this medication. Altrenogest is readily absorbed through skin contact and has potent hormonal activity. Pregnant women must not handle the drug under any circumstances due to risk of fetal harm. Women who may become pregnant or are of childbearing age should avoid handling altrenogest or must wear impervious protective gloves and wash thoroughly after any potential exposure. Accidental skin contact requires immediate thorough washing with soap and water. All handlers, regardless of gender, should wear protective gloves during administration and avoid any skin contact with the medication. These precautions must be clearly communicated to all individuals involved in the mare's care.

Monitoring requirements during altrenogest therapy include observation for expected therapeutic effects and awareness of potential complications. Owners should note whether estrus behaviors are adequately suppressed and report to the veterinarian if signs of heat continue despite appropriate dosing. For pregnant mares receiving altrenogest supplementation, regular veterinary monitoring of pregnancy viability through ultrasound examination is standard practice. Any signs of reproductive tract abnormalities, discharge, or pregnancy complications warrant prompt veterinary evaluation. General health monitoring should continue throughout therapy, with attention to any changes in appetite, attitude, or physical condition.

Competition considerations for altrenogest use require attention to current regulations under the relevant governing body. FEI (Fédération Équestre Internationale) and USEF (United States Equestrian Federation) currently permit altrenogest use in mares with appropriate documentation, but rules may vary by competition level, country, or discipline. Some national federations prohibit altrenogest use entirely. Racing jurisdictions typically prohibit the medication. Verification of current regulations is essential before competing, as rules evolve and non-compliance can result in disqualification and penalties. Maintaining accurate medication records and being prepared to provide documentation upon request protects both mare welfare and competitive standing.

Environmental and disposal precautions apply to altrenogest due to its hormonal activity and potential ecological effects. Unused medication should be disposed of properly according to local pharmaceutical waste guidelines rather than discarding in regular trash or flushing down drains. Contaminated materials such as used gloves should be disposed of appropriately. The oral solution container should be kept tightly closed when not in use to prevent spills and maintain medication stability. Any spilled medication should be cleaned up carefully using appropriate personal protective equipment.

Long-term use considerations include periodic reassessment of continued need for therapy. Mares on prolonged altrenogest treatment should be periodically evaluated to confirm ongoing indication and absence of emerging health issues. The effects of years-long continuous altrenogest use on mare reproductive health are not fully characterized, and periodic cycling off medication may be recommended for mares not actively competing or when practical. Cost-benefit analysis for long-term therapy should account for medication expense, handler safety burden, and therapeutic benefit. Alternative management strategies such as surgical options (ovariectomy) or intrauterine devices may be considered for mares requiring permanent estrus suppression.

Proper storage of altrenogest is essential for maintaining medication efficacy and ensuring handler safety. The oral solution should be stored at controlled room temperature between 68°F and 77°F (20°C to 25°C), protected from excessive heat, cold, and light. In barn environments, this means storing altrenogest in a climate-controlled area rather than in tack rooms subject to temperature extremes. The medication container should remain tightly closed when not in use to maintain product stability and prevent accidental exposure or spillage. Avoid storing near feed or in areas where the container could be knocked over or accessed by children or unauthorized persons.

Handler safety during storage and administration of altrenogest requires strict adherence to protective protocols. The medication container and dosing syringe should be handled only by individuals aware of the risks and appropriate precautions. Impervious gloves must be worn whenever opening the container, filling the dosing syringe, or administering the medication to the mare. Nitrile or other chemical-resistant gloves provide appropriate protection; latex gloves may not be adequate. After handling, gloves should be carefully removed to avoid contaminating bare skin and disposed of appropriately. Hands should be thoroughly washed with soap and water even after gloved handling. Any skin contact requires immediate washing and should be reported to a healthcare provider if significant exposure occurs.

Specific precautions for women are critical and must be emphasized. Pregnant women must never handle altrenogest due to risk of fetal harm. Women who may become pregnant or are of childbearing age should avoid handling the medication whenever possible. If handling is unavoidable, double gloving and extreme care to prevent any skin contact are essential. Women who have inadvertent skin exposure should consult a healthcare provider promptly. Barns and facilities using altrenogest should have clear protocols identifying who is permitted to handle the medication and ensuring appropriate training for all authorized handlers.

Expiration dates on altrenogest containers must be observed, as the manufacturer's dating ensures potency and stability. Using expired medication may result in inadequate estrus suppression. Once opened, the product should be used within the timeframe specified on the label, as stability after opening may be limited compared to sealed containers. Signs of product degradation such as changes in color, consistency, or odor indicate the medication should not be used. Proper disposal of unused or expired altrenogest involves following local guidelines for pharmaceutical waste with hormonal activity. The medication should not be poured down drains, disposed of in regular trash, or otherwise released into the environment where hormonal contamination could affect water supplies or wildlife. Return to a pharmacy or veterinary clinic for proper disposal is the preferred approach.

Draft mares receiving altrenogest therapy require appropriate dose calculation for their substantial body mass. Breeds such as Clydesdales, Percherons, Belgians, and Shires commonly weigh between 1,600 and 2,200 pounds, requiring proportionally larger daily doses. The volume of oral solution needed for draft mares is significant, and ensuring complete administration of the full dose requires attention to technique. Draft breed mares may be used for driving, draft work, or pleasure riding where estrus-related behaviors could create safety concerns or interfere with work performance. The generally calm temperament of draft breeds may mean extreme behavioral issues during estrus are less common than in hotter types, but individual draft mares certainly can benefit from estrus suppression when indicated.

Light horse breeds and warmbloods represent the primary population receiving altrenogest in equine practice. Performance mares in disciplines including dressage, show jumping, eventing, hunter/jumper, reining, and many others commonly receive altrenogest to manage estrus-related performance issues. The competitive demands on these mares often conflict with natural cycling behaviors, making estrus suppression valuable for consistent training and competition results. Thoroughbred and Standardbred mares in racing may receive altrenogest under jurisdiction-specific regulations, though racing rules often prohibit or restrict hormone use. Arabian mares, Quarter Horse mares, and mares of other light breeds respond predictably to standard altrenogest dosing based on body weight.

Ponies and miniature horse mares require careful dose calculation to ensure appropriate therapy. These smaller equines need proportionally reduced doses based on their body weight, and the convenient per-50-kg dosing of the oral solution formulation facilitates accurate dosing across the size spectrum. Pony mares may exhibit particularly dramatic estrus behaviors due to their often more assertive temperaments, making them good candidates for behavioral management with altrenogest when indicated. Miniature horse mares used as companion animals or for showing may benefit from estrus suppression if cycling creates management challenges. Accurate weight determination through scale measurement is valuable for precise dosing in smaller equines.

Breed-specific reproductive considerations may influence altrenogest use in certain populations. Some breeds have documented higher rates of early embryonic loss, making pregnancy supplementation with altrenogest particularly relevant. Mares with breed-specific uterine or hormonal conditions require comprehensive reproductive evaluation when estrus-related issues arise. Friesian mares have been reported to have higher rates of retained placenta and certain reproductive complications, warranting attention to overall reproductive management beyond estrus suppression alone. For valuable breeding mares of any breed, the decision to use long-term altrenogest for behavioral management must balance competitive benefit against any theoretical concerns about prolonged hormone therapy effects on future fertility, though no documented adverse fertility effects from altrenogest use have been established.

Within the progestogen class, other preparations may occasionally be used for estrus suppression or pregnancy support in mares. Progesterone in oil, administered by injection, provides an alternative delivery route and was commonly used before altrenogest's development. Compounded progesterone preparations exist but vary in quality and reliability. Long-acting progesterone formulations may be available in some markets. The advantages of altrenogest over injectable progesterone include oral administration convenience, consistent formulation quality, and established dosing protocols. Medroxyprogesterone acetate (MPA) and other synthetic progestogens have been explored but are not commonly used in current equine practice. The choice among progestogen options typically favors altrenogest due to its FDA approval, extensive use history, and convenient formulation.

Alternative approaches to estrus suppression include GnRH-based treatments that work through different mechanisms. Deslorelin acetate implants (Ovuplant) can suppress ovarian activity when administered in appropriate protocols, though this represents off-label use for behavior modification. Intrauterine devices, specifically glass marbles or specialized equine intrauterine devices, can mimic pregnancy and suppress estrus in some mares through physical presence in the uterus. This mechanical approach avoids ongoing medication costs and handler exposure risks but requires proper placement and monitoring. Ovariectomy (surgical removal of the ovaries) provides permanent estrus elimination for mares not intended for breeding and may be appropriate for mares with severe behavioral issues unresponsive to medical management.

Complementary approaches and management strategies may work alongside altrenogest or serve as alternatives for mares with mild symptoms. Nutritional supplements marketed for mare temperament, including various herbal preparations and specific nutrient combinations, are widely used though scientific evidence for efficacy is limited. Acupuncture has been explored for behavioral modulation in horses with variable results. Management modifications such as exercise adjustment, social grouping changes, and environmental enrichment may help some mares. Understanding that not all difficult mare behavior is hormonally mediated is important, as behavioral issues with other causes will not respond to altrenogest therapy. Comprehensive evaluation by a veterinarian helps identify the most appropriate management approach for each individual mare, potentially including behavioral consultation for complex cases.