When Good Drugs Do Bad Things

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Introduction | Drugs for pain and inflammation | Steroids | Tranquilizers/sedatives | Dewormers | Antibiotics | Joint injections | Reporting adverse drug reactions | About the Author


The development of antibiotics, pain killers and numerous other drugs has improved the quality of health care for horses. But sometimes, administering a drug to a horse unintentionally results in disastrous consequences. Adverse drug reactions are uncommon, but horse owners should be aware of some the more frequent reactions. Always read the label and follow the directions on any drug package or bottle. Keep good records of medications that you use in your horses. Always consult your veterinarian if you have any concerns or questions.

Drugs for Pain and Inflammation

The most common drugs given to horses to relieve the pain and inflammation of lameness and colic are the non-steroidal anti-inflammatory drugs (NSAIDs). All of these drugs work by blocking prostaglandins - chemicals that cause the symptoms of pain and inflammation. Unfortunately, there are also "good" prostaglandins that maintain blood flow to the kidneys and the lining of the stomach and intestines. So all of these drugs have the potential to cause kidney damage and ulcers in the mouth, stomach and intestines. The risk of toxicity is greater in foals and old horses, and in horses who are dehydrated. Clinical signs of toxicity include teeth grinding and drooling, low grade colic pain, diarrhea and fluid accumulation on the abdomen and legs. Kidney failure and perforation of stomach and intestine ulcers can be deadly. Also, most of these drugs are acid formulations and may be very irritating to muscle tissues when given as injections in the neck muscles or hindquarters.

Phenylbutazone, usually known as "bute", is very commonly used for pain and inflammation in horses because it is effective and inexpensive. However, this drug remains in the horse's bloodstream for a long time after each dose, so it is the most common NSAID to cause toxicity. Bute should be given at the lowest possible effective dose and on a once a day basis. For long term use, it's best to give a low dose every other day. Bute is available in paste and tablets for oral administration and as an injectable product for intravenous administration. If given outside of the vein, bute is extremely irritating to the horse's muscle tissues.

Banamine®(flunixin meglumine) is considerably more expensive than bute, so it is usually reserved to treat colic pain on a short term basis. It does not stay in the horse's bloodstream as long as bute, so it is less likely to cause toxicity. But high doses, especially in foals or dehydrated horses, can still cause kidney and digestive tract toxicity. Banamine® is available as oral granules and as an injectable formulation. The injectable product is labelled for intramuscular or intravenous use. I highly recommend that Banamine® only be administered by intravenous injection. The Banamine® formulation causes significant muscle damage when injected, which may result in a "flesh eating" infection by clostridial bacteria. On rare occasions, clostridial bacteria are picked up from the horse's haircoat as the injection needle passes through into the muscle. These bacteria only grow in an environment without oxygen. The muscle damage from the Banamine® formulation provides this perfect environment. Clostridial infections in the muscles are life-threatening. Finding a swollen, painful area that is cool to the touch and has the crackling feeling of gas bubbles under the skin is an indication of this very serious infection.

Dipyrone is an old drug that is very good at reducing a fever, but is not a particularly powerful anti-inflammatory drug or pain killer. It is not frequently used anymore, but some veterinarians may still provide it for horse owners to have on hand for colicking horses. This product is labelled to be administered intravenously, subcutaneously or intramuscularly. However, this formulation is extremely irritating to muscle tissues when given intramuscularly.


There are two major categories of steroid drugs: corticosteroids and anabolic steroids. The corticosteroids are used for relieving inflammatory conditions while the anabolic steroids are used to build muscle and encourage appetite.

Corticosteroids are available in many different brands and formulations (Azium®, Depo-Medrol®, prednisone, Flucort®, etc). They are powerful drugs for relieving allergic reactions, itchy skin problems and attacks of chronic obstructive pulmonary disease (COPD or "heaves"). The steroids drugs supplement the body's own natural steroids, and influence many tissues in the body. Steroids are not widely used in horses because they are associated with causing laminitis (founder). This is difficult to prove with a scientific study, but equine veterinarians can relate many experiences with this reaction. It seems to be more common with the steroids that are given by injection than by those given orally. Some steroid formulations are directly injected into joints to relieve lameness. This method avoids the overall systemic effect of the steroid, but can cause local problems. When steroids are injected into joints, the horse may improve so much that he is put back into work before the joint has healed and further joint damage may occur. Also, steroids decrease the normal function of the horse's immune system. If bacteria are introduced into the joint with the injection, this can lead to a disastrous joint infection. Steroids are often combined with antibiotics in eye ointments. This combination is for minor irriation, but should never be used if there is actual damage to the horse's cornea. Steroids will slow the healing rate of a damaged cornea and interfere with normal defense mechanisms against infection. A simple corneal ulcer can rapidly progress to a severe sight-threatening infection if steroids are applied. Eye injuries in horses are very serious - always consult your veterinarian before putting any medication in a horse's eye!

Anabolic steroids (Equipoise®, Winstrol V®) are labelled to be used in horses that are in poor body condition because of disease or overwork. But because they increase appetite and muscle mass, they are frequently used to "bulk up" show horses. These drugs are all chemically related to testosterone. When given to stallions or mares, these drugs can interfere with normal sexual functions. Some of the affects will reverse when the drugs are no longer administered, but some animals will remain permanently infertile. In some horses, the testosterone effect will dramatically affect their temperaments - turning normally placid horses into "land sharks". These effects may last for 6 to 8 weeks.


Acepromazine (Atravet®, Acepro-25, etc) is a tranquilizer that is available at feed stores. I personally think that this drug should only be available through a veterinarian as it must be dosed very carefully. It has no effect on pain sensation, so a tranquilized horse may react violently to a painful procedure. Acepromazine dilates blood vessels and lowers blood pressure, so should not be administered to horses that are bleeding profusely or are in shock. In male horses, a rare reaction to acepromazine results in paraphimosis - involuntary extension and erection of the penis. The horse's penis is easily damaged in this state.

Fluphenazine (Prolixen®) is an extremely long-acting cousin of acepromazine that is used in severely psychotic people and is being used in performance horses. Its effects last for months - long after any traces of the drug are gone from the horse's blood or urine. Even though this drug is undetectable by testing agencies, such use to alter performance is certainly unethical. On rare occasions, this drug has a paradoxical effect. Approximately 12 hours after injection, a horse may suddenly go berserk and may seriously injure itself or people around it. The only antidote for this reaction is an intravenous injection of antihistamine - a difficult thing to do in a horse that is freaking out.

Rompun® (xylazine), Dormosedan® (detomidine), and Sedivet® (romifidine) are related sedative drugs. Unlike acepromazine, these drugs all relieve pain, so they are commonly used for colicking horses or to restrain a horse for a painful procedure. Pain control is greater in the horse's front end, so care must be used when performing procedures on the horse's hind end. Normally well-behaved horses will lose their inhibition to kick. These drugs all lower blood pressure and slow the heart rate, so must be used carefully in a horse in shock.

Horses who are already excited may not respond adequately to any of the tranquilizers and sedatives. And even if apparently sedated, a horse may "override" the affect of the drug if startled or a procedure is painful. All tranquilizers and sedatives must be given very carefully by intravenous injection. If the drug is injected into the horse's carotid artery instead of the jugular vein, the drug will go directly to the horse's brain. This results in a violent reaction - the horse will throw itself over backwards and often will seizure. Most horses recover as long at they don't injure their skulls or spines when reacting.


Quest® (moxidectin) is a new dewormer that is a cousin to ivermectin (Eqvalan®). The drug is less active against bots than ivermectin, so the paste formulation is twice the concentration as ivermectin. Moxidectin is distributed to body fat more than ivermectin. Since ivermectin is so safe, horse owners commonly administer an entire tube of the paste no matter what the size of the horse. Since its release as a dewormer, Quest® has been involved in a number of drug reactions. Despite the young foal on the package, Quest® is not labelled for use in foals under 4 months of age. Foals have very little body fat, allowing higher levels of moxidectin in the blood stream, which then crosses into the brain and causes staggering, weakness, coma and even death. Also, the tube of Quest® is labelled to treat 1150 lbs. Most horse owners overestimate their horse's weight and often give the whole tube no matter what the horse weighs. Weight tapes are readily available and should be used before dosing any horse with Quest®. However, it is very difficult for owners to get accurate weights on thin horses, miniature horses, donkey and mules, and these equines have been involved in a lot of the problems with Quest®. When dosed accurately, Quest is a very good dewormer.

The other dewormers currently available are very safe when used as directed. Care should always be take in administering such drugs to very young, very old or ill horses. Consult a veterinarian for proper use of such products.


All antibiotic drugs have the potential to cause diarrhea (colitis) in horses. Horses have a very delicate digestive tract - it appears to have been designed by a committee! Antibiotics kill bacteria, and our goal is to use them to kill abnormal bacteria that are causing an infection. However, there are normal bacteria that live in the horse's digestive tract that help in the digestion of the horse's roughage diet. Unfortunately, antibiotics can not distinguish the "bad" bacteria from the normal inhabitants of the intestines. The risk of antibiotic-induced diarrhea dramatically increases when a horse receives more than one antibiotic at a time. Most of the time the diarrhea is self-limiting and resolves when the antibiotic is stopped and with supportive care. Occasionally, the diarrhea can be severe enough to be life-threatening.

There are some drug reactions specific to certain antibiotics. Just like people, horses can develop allergic reactions to penicillin. This requires previous exposure to the drug, and then is manifested by hives and head swelling, and occasionally, massive constriction of the airways and sudden death. Penicillin is very commonly administered to horses in a formulation known as penicillin procaine G. The procaine is a local anesthetic, related to lidocaine, novacaine and cocaine! It helps to give the formulation a long-acting effect, so that it can be administered only twice a day. When correctly injected into the horse's muscle, it causes no problem. However, if the formulation is accidentally injected into the horse's bloodstream, the procaine goes to the horse's brain and triggers a spectacular reaction (imagine a horse getting a "hit" of cocaine). Most horses begin to tremble violently and throw themselves over backwards. There is no antidote when this occurs and an affected horse will recover in just a few minutes as long as it doesn't damage its skull or spine. Obviously, this reaction can be very dangerous for bystanders. This is why when injecting penicillin into a horse's neck or hindquarters, you should always place the needle first, and watch for blood to fill the hub of the needle. If you see blood, remove the needle and place a clean needle in another site. When you attach the syringe full of penicillin, pull back and again watch for blood to appear in the syringe. Never follow through with the injection if you see blood. Also, the procaine in the formulation becomes more soluble at higher temperatures, increasing the risk of it entering a blood vessel. Therefore, penicillin procaine G should always be kept refrigerated until just before injecting. Very rarely, penicillin can cause the horse to destroy its own red blood cells and the horse shows signs of severe anemia. This reaction usually resolves by stopping the penicillin therapy and giving supportive care.

Trimethoprim/sulfas (Tribrissen®, Bactrim®, etc) are an antibiotic combination that can be given intravenously or orally to horses. The oral tablets or paste are commonly given to horses to avoid having to give injections of penicillin. The intravenous formulation must be given slowly, as rapid administration can cause the horse to collapse. Intravenous trimethoprim/sulfas should not be given at the same time as Dormosedan® (detomidine), as a this combination has been documented to cause fatal heart attacks.

Joint Injections

Athletic horses are very prone to developing arthritic joint problems. A number of drugs are used to treat these conditions, and some are injected directly into the affected joint. Steroids, Adequan® and hyaluronic acid (HA) products can be used this way. Although these drugs are anti-inflammatory drugs, on rare occasions, the horse's joint may react to the presence of the foreign material. This shows as a rapidly developing increase in the horse's lameness and the joint may be hot and swell considerably. This reaction usually resolves in a few days with supportive therapy. Unfortunately, in the injection process it is possible to pick up bacteria on the horse's skin and inject them along with the drug into the horse's joint. A bacterial injection of the joint initially looks exactly like the drug reaction, only it is a far more serious situation. If not treated very promptly and aggressively, the bacterial infection can destroy the horse's joint. So when a horse's joint blows up after a joint injection, this is a nerve-wracking situation for the veterinarian and owner, as the drug reaction will subside in a day or two, but if you wait with a joint infection, the horse's athletic career may be over. For this reason, Adequan® and the hyaluronic acid brand known as Legend® come in alternative formulations. Adequan can be injected intramuscularly and Legend® can be given intravenously, altogether avoiding the risk of a joint infection.

Reporting Adverse Drug Reactions

As a profession, veterinarians need to know more about adverse drug reactions. When such reactions occur, it is important to report them. The best place to have your veterinarian report reactions is the USP Veterinary Practitioners Reporting Program by calling 1-800-4-USPPRN.

About the Author

Dr. Trish Dowling is one of two people board certified in medicine and pharmacology. She teaches clinical pharmacology at the Western College of Veterinary Medicine in Saskatoon. Trish has competed in three-day events, endurance and competitive trail rides. She also has an interest in soundness of horses.

This information was presented at, and appears in the Proceedings of, the 1999 Alberta Horse Breeders and Owners Conference.

This information is maintained by of the Horse Industry Section of Alberta Agriculture in conjunction with Sylvia Schneider at Pondside Web Productions.

Patricia M. Dowling, DVM, MS, Dipl. ACVIM, Dipl. ACVCP
Western College of Veterinary Medicine
University of Saskatchewan
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This information published to the web on March 13, 1999.
Last Reviewed/Revised on July 5, 2016.