Therapeutic drugs such as antibiotics, anti-inflammatory and pain relief medications, anesthetics, and antiparasitic drugs can dramatically improve the health and well-being of horses. Without these drugs, many horses would die or suffer serious consequences from conditions that might otherwise pose minimal risks. However, many horse owners are unaware that virtually all drugs can cause unintended side effects, or adverse effects, that sometimes can be serious.

Some drugs are safer than others with little risk of serious side effects even with marked overdoses. Other drugs have much narrower margins of safety and can cause major side effects at normal therapeutic doses. Many factors influence whether side effects will occur and how severe they are. These factors include species, individual variability, age, pre-existing medical conditions, other drugs given concurrently, pregnancy, and drug dosage, among others.

For example, some drugs

  • are safe for cattle are very dangerous to horses;
  • are more dangerous to foals compared to adult horses due to differences in metabolism and clearance of the drug from the body;
  • can be dangerous in dehydrated animals or animals with kidney impairment; and 
  • should not be used in pregnant mares due to risk of abortion or adverse effects on the fetus.

Examples of side effects that can be associated with common therapeutic drugs in horses include: gastrointestinal disturbances such as colic and colitis secondary to antibiotic therapy; gastrointestinal ulceration and kidney damage associated with the use of some anti-inflammatory drugs (e.g., banamine and phenylbutazone); and laminitis secondary to use of glucocorticoids (e.g., dexamethasone).

Many adverse drug effects are well-documented and can be anticipated. Others, called idiosyncratic reactions, are unexpected, usually rare, and are not dose-dependent. Horse owners should always consult with their veterinarians before using any drug, even common drugs such as phenylbutazone. Veterinarians are familiar with anticipated side effects and the factors that can influence risk of toxicity from the drug. Your veterinarian can advise on the appropriateness of drug use and recommend the best dosing regimen that will reduce the risk of unintended side effects.

Therapeutic drugs play an important role in keeping horses healthy and comfortable, so horse owners should not be discouraged by the potential for side effects. Your veterinarian can help determine whether the risks from the drug outweigh the benefits. Be sure to ask your veterinarian about any potential side effects whenever using therapeutic drugs.

Cynthia Gaskill, DVM, PhD, clinical veterinary toxicologist at the University of Kentucky Livestock Disease Diagnostic Center, provided this information.

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