Human Chorionic Gonadotropin for Horses: Uses, Dosing & Side Effects

This information is for educational purposes only. Never give your pet any medication without your veterinarian's guidance. Dosing, frequency, and safety depend on your pet's specific health profile.

Human chorionic gonadotropin, usually shortened to hCG, is a hormone medication your vet may use in mares to help trigger ovulation at a more predictable time. In equine practice, it is used mainly for reproductive management rather than for general illness treatment.

Although the drug comes from human medicine, it has biologic activity similar to luteinizing hormone in the mare. That is why it can help a mature follicle ovulate when the mare is already in estrus and close to ovulation. In the United States, hCG is commonly used extra-label in horses, which means your vet is using a legal veterinary prescribing pathway for a species not listed on the product label.

For many breeding programs, timing matters. A mare may need insemination coordinated with shipped semen, frozen semen, stallion availability, or embryo transfer plans. In those situations, hCG can be one option your vet considers alongside other reproductive tools such as deslorelin.

This is not a medication pet parents should try to source or give on their own. Proper use depends on ultrasound findings, cycle stage, breeding goals, and the mare's reproductive history.

The main use of hCG in horses is inducing ovulation in mares that are already in heat and have a mature preovulatory follicle. Merck notes that mares with follicles larger than 30 mm may be candidates, and another Merck equine reproduction reference describes common use when a follicle is at least 35 mm and the mare is close to ovulation.

In practical terms, your vet may use hCG to help narrow the expected ovulation window to about 36 to 48 hours. That can make breeding management more efficient, especially when semen shipment timing, stallion booking, or repeated farm calls are concerns.

It is not a fertility cure-all. hCG does not fix every cause of infertility, and it is not a substitute for a full reproductive exam. If a mare has uterine disease, poor uterine clearance, transitional follicles, or other breeding problems, your vet may recommend a different plan or combine hCG with monitoring and additional reproductive care.

Some reproductive specialists may also weigh hCG against deslorelin. Both can be used to induce ovulation, but the best fit depends on the mare, the breeding schedule, prior response, and how intensively the cycle is being managed.

hCG dosing in horses should be determined by your vet after palpation and ultrasound. A commonly cited equine dose is 2,500 IU IV for mares with a mature preovulatory follicle. Merck also notes extra-label use at 2,500 to 5,000 IU IV or IM to induce ovulation in 36 to 48 hours when a follicle at least 35 mm is present.

AAEP proceedings describe typical single-bolus doses of about 1,500 to 3,300 IU, with many equine practitioners using 2,000 to 2,500 IU from a 10,000-IU vial. In general, this is given as a single dose, not a repeated daily medication. More is not always better, and route matters. Some equine clinicians prefer IV administration, and unnecessarily high IM doses may be avoided.

Timing is everything. hCG works best when the mare is truly in estrus, has uterine edema consistent with heat, and has a viable growing follicle that is already near ovulation. If the follicle is not ready, the mare is transitional, or the cycle stage is misread, the response may be less predictable.

Because this is a reproductive drug used around a narrow breeding window, your vet may pair the injection with serial ultrasound exams before and after treatment. That monitoring often matters as much as the drug itself.

hCG is generally used because it can be effective and practical, but it is still a prescription hormone and should be treated with care. Published equine references focus more on variable response over time than on a long list of dramatic side effects in mares.

The most important caution is that repeated use over a long period can lead to antibody formation, and that may reduce how well the mare responds in future cycles. In other words, a mare that responded well earlier in the season may become less predictable after repeated exposure.

As with many injectable medications, mild injection-site irritation or a short-lived local reaction is possible. Rare hypersensitivity reactions are a general concern with biologic products, so your vet will consider the mare's history and monitor as needed after administration.

Call your vet promptly if the mare seems painful after injection, develops swelling, hives, breathing changes, marked agitation, or fails to ovulate as expected. A missed response is not always an emergency, but it does change breeding timing and may require a different plan.

There are no widely cited everyday drug interactions for hCG in horses like you might see with antibiotics or pain medications. The bigger issue is how hCG fits into the mare's overall reproductive protocol.

Your vet may use hCG alongside or in sequence with other reproductive medications such as deslorelin, prostaglandins, progesterone-based protocols, estrogens, or altrenogest, depending on the breeding goal. These combinations are not necessarily unsafe, but they do change timing, expected follicle response, and interpretation of ultrasound findings.

Because hCG is used to influence ovulation, it should not be added casually to a mare already receiving other hormone therapies. The same mare may need a very different plan if she is cycling normally, in spring transition, being synchronized for embryo transfer, or being managed after a prior poor response.

Tell your vet about every medication, hormone, and supplement the mare is receiving, including recent reproductive treatments. That helps avoid protocol conflicts, mistimed breeding, and unnecessary repeat dosing.