Equine Drugs and Medication Part 2



“What is the drug called Estradiole Cypionate used for ?”…

AAH Staff’s Answer:


Estradiol is the most active of the endogenous estrogens. Estradiol cypionate (ECP) is an esterified estrogen that is more potent and has a longer half-life than the predominant naturally occurring estrogen, estradiol 17 beta. The actions of estrogen hormones on the female include maturation of the reproductive tract and mammary glands, modulation of pituitary gonadotropin secretion, expression of estrus behavior, enhanced uterine resistance to infection, gestational support during pregnancy, and modulation of uterine contractions and cervical relaxation during labor.

ECP is most commonly used in mares in order to enhance sexual receptivity in ovariectomized mares being used as stimulus mares in a semen collection program. ECP may also be used in a seasonally anestrous mare to produce sexual receptivity for the same purpose. Although the anestrous mare may be sexually receptive due to the effect of the ECP, if she is truly anestrous, she will of course not produce follicles or ovulate. ECP or other estrogens will not produce sexual receptivity in mares that are in diestrus with circulating progesterone.

Other less common uses for ECP in the mare include:

  • Increasing sexual receptivity in mares that are in physiological estrus but are still not receptive to the stallion.
  • Silent heats” can be a real management headache when natural service is necessary. The administration of exogenous estrogen has been tried under these circumstances, but research has not been done in the horse to prove whether or not there are any adverse effects on conception rates.
  • Some labor induction protocols include the use of ECP to soften the cervix prior to the administration of oxytocin.
  • Some veterinarians use exogenous estrogens on mares while treating uterine infections because research shows that the uterus is more effective at clearing infection while under the influence of estrogen.
  • Some researchers are working with exogenous estrogens or progesterone/ estrogen combinations for pregnancy maintenance in mares that habitually suffer early embryonic loss.
  • Many of these treatments are still in the research stages and the most recent information should be consulted before embarking on treatment.

Side Effects

Injection site reactions may occur. No other side effects have been reported when ECP is used in ovariectomized mares.


Estrogens are generally contraindicated in pregnant mares. As noted above there is some research on their uses in mares with a history of early pregnancy loss but our understanding of this process is still evolving.


Drug Interactions

Rifampin, phenobarbital and phenylbutazone may decrease estrogen activity. The effects of glucocorticoids may be enhanced when used with estrogens. The dose of corticosteroids may need to be reduced.


“I lost two other horses due to salmonella poisoning the last time he shed it”…


I have a 2yr old who had pneumonia when he was 6mth old he shed salmonella from the strong antibiotics he took, is it likely that he will shed it again? He was at the trainer and just came home. His stool is normal at first and then it turns loose (a little thicker than cows). He does not act sick at all. We did worm him in the last three days would this cause the loose stool? I lost two other horses due to salmonella poisoning the last time he shed it. Is it possible he will never shed it again and if he does will it be at a toxic level to the other horse in the pasture since he has not been on any more antibiotics since then. Thank you Hillari


Dr. Kimberly Gryl’s Answer:



I have a hard time following your question, but I will give you the basics on Salmonella shedding.  Any time an animal is on antibiotics, there is potential for the manure to become loose due to altered numbers or imbalance of gastrointestinal bacterial.  There is a ton of bacteria that live in the intestine in a normal, healthy horse.  When these numbers become more or less, loose manure may be seen.  When this happens, there is potential for fecal shedding of “bad” bacteria, such as Salmonella.  There is no limit to the number of times that Salmonella (or other bad bacteria) can be passed in the manure, so just because he shed it once before does not mean that he can or can’t, or will or won’t, shed it again in the event of loose manure.

Any time you see loose manure a horse that is turned out with other horses, it is a good idea to isolate the horse with the loose manure.  This is to prevent passage of the “bad” bacteria, toxins, or other possible contaminants from the sick horse to the other (presumably healthy) horses.  Like you experienced, other horses can get sick from passage like this, and can die from it.  I hope you never have to go through that again.

If you ever have a different veterinarian evaluate your horse and prescribe antibiotics, it might be worth your while to let him or her know of this past experience.  Sometimes that may alter the antibiotic choice.  Either way, it can then be noted in the horse’s medical record.


“What are the effects of tranquilizers on the mare and foal if we decided to AI…?”
Last year we bred our mare, who sometime over the winter slipped the foal. The mare is hard to handle for a lot of procedures, we did live cover when breeding. Now that she is no longer in foal the stud owner is only doing AI my vet says for this particular mare, live cover is best, as in order to preform AI we would have to tranc her for every procedure, what are the effects of tranquilizers on the mare and foal if we decided to AI? Are there any known complications? Thank you

Dr. Jack Sales’ Answer:

Hi Cheryl,

The use of a tranquilizer in order to Artificially Inseminate (AI) your mare would be safe to the mare and the fertilization process. I would think that your mare would only have to be tranquilized 2 or 3 times at the most. They would probably decide to breed your mare every other day while she is in heat starting with the second or third day of heat. The AI procedure is usually well tolerated by a mare in heat, so it may be possible that your mare not actually need to be tranquilized. I am sure that they would use the minimal amount of restraint necessary.


“Reserpine and Fluphenazin…”

I am interested in finding out more information and what you thought about the two products. I have two horses that I use for barrel racing and the get overly nervous ( out of control) when training or competing. I consider myself a patient and slow trainer, but their behavior gets very hard to deal with, even with the simple things like waiting to make a run. Please let me know what your thoughts are on this subject.

Dr. Jack Sales’ Answer:

Hi Hazen.

Reserpine has been used successfully on many horses for the long term management of overly nervous horses. It is a human drug that is used to lower the blood pressure in humans. In the horse it has been found to be helpful when given in a single dose, to calm a flighty or nervous horse for sometimes up to 10 days to 2 weeks. Then it must be repeated. Overdose and some horses on a normal dose of 2 to 3 cc will develop diarrhea, so it is not without some risk. Also, not all horses seem to respond to it as well as others. Fluphenazine, on the other hand, although more expensive, is a little more predictable, since it is an actual human tranquilizer. It will usually work long term on horses with just one dose, but all horses seem to respond a little bit differently to it. Trial and error is the best that can be expected from these products since they are not actually federally okayed for use in horses. Depend on your Veterinarian and his advice if you decide to try either of these products.


“What is the efficiency for the WNV vaccine…”

What is the efficiency for the WNV vaccine and what are the side-effect risks with the WNV vaccine?

Dr. Jack Sales’ Answer:

In answer to your question regarding the West Nile Virus vaccine and its effectiveness, I would say that although there have not been many detailed studies regarding the effectiveness, risks, or side effects of this vaccine, because it is so new, I think that we can assume that it is going to be very similar to the effectiveness, risks and side effects of the Sleeping Sickness vaccines that are on the market and used routinely today. That is the Eastern, Western, and Venezuelan forms of encephalitis. The West Nile Virus is very similar to these other viruses and the company that made the West Nile Virus vaccine, also have the experience of formulating and manufacturing these more common vaccines in use. (EEE, WEE, & VEE) To date, there have been no reports regarding any serious side effects or risks with the use of this new WNV vaccine.


One Response to "Equine Drugs and Medication Part 2"
  1. Myra says:

    Considering using Estradiole Cypionate as a behavior/hormone therapy for my 3 year old stallion during show season. What are your thought or other solutions?


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