Equine Drug Interactions

Drug and medication interactions

Drugs and medications affect the horse's bodily activities and are helpful in reducing pain, preventing and curing disease, and enhancing the horse's general health.  Unfortunately, during and after administration, drugs and medications can interact with each other in ways that may be harmful or dangerous when improperly prescribed or used.

Combining drugs and medications

Combining drugs and medications for treatment is common, but care must be taken to avoid dangerous interactions.

Given the number of drugs and medications available for use with horses, it is little wonder that prescription and dosage errors occur. Awareness of possible interactions can help the horse owner take positive steps by recognizing when an interaction is taking place, and, also can help in the prevention of such occurrences.

Drug and medication interactions may occur because of the following prescription errors:

• Prescribing a combination of incompatible drugs
• Prescribing a drug or medication that results in an allergic reaction
• Prescribing an incorrect dosage of a drug or medication
• Prescribing an incorrect administration of a drug or medication such as IV as opposed to IM
• Prescribing a drug or medication for prolonged use without consideration of adverse effects on the health of the horse
• Prescribing a drug or medication that interacts in a negative way with the horse's diet
• Using a drug or medication that is outdated or expired

In addition drug and medication errors are sometimes compounded by a pharmacist misreading a prescription, mixing up drugs with similar names, or misreading or misinterpreting the prescription dosage.

Horses react to drugs and medications in much the same way as humans. Any time a horse is treated with chemical/medicinal products of any kind, going through a change in diet, or receiving drugs or medications via a different route, care must be taken by the veterinarian and owner to make sure that interactions do not occur, and, if they do, quickly working to minimize any negative effects.

Common drug and medication interactions

• Analgesics/Sedatives: may have additive effects when combined with other analgesics, sedatives, tranquilizers and general anesthetic drugs. May have adverse affects when used with intravenous sulfonamide drugs.
• Antibiotics: when combined with diuretics, antibiotics increase chances of toxicity to the ear, kidneys and nervous system and can increase the effects of drugs used during general anesthesia. Can also interact negatively or cause allergic reactions, interfere with prostaglandin synthesis, and produce undesirable inflammatory products and enzymes.
• Anti-histamines: have an additive effect when combined with other central nervous system depressant drugs and may affect the actions of anticoagulants such as warfarin.
• Anti-inflammatories: non-steroidal anti-inflammatory drugs (NSAIDs) may cause ulcers when over used or combined with other anti-inflammatories and can interact with anti-coagulant drugs especially coumarin derivatives. Used with furosemide, excretion of aspirin via the kidneys is slowed.
• De-wormers: may contain chemicals known to cause liver toxicity especially when given with duretics, sulfa preparations, anti-convulsants and some steroids, and may contain chemicals that may cause abortion.
• Prostaglandins: may increase activity of other drugs that affect the mare's uterus such as oxytocin.
• Steroid hormones: may increase risk of electrolyte imbalances due to calcium and potassium losses, and may increase risk of gastrointestinal ulcers when given along with NSAIDs. Immune response to vaccination may be reduced if corticosteroids are given at same time.
• Vasodilators: should be used with caution when given with drugs that affect blood pressure including most sedatives and drugs used for general anesthesia.

This list of drug interactions is in no way complete. For information regarding specific drugs and medications, click on the name of the drug in the Drugs and Medications list or contact your veterinarian for more information.

The best way to prevent adverse drug and medication interactions is to have good communication with your veterinarian.  Always double check dosage and method of administration and give all medications exactly as prescribed in terms of dose, dosing interval and duration of treatment. Monitor your horse's condition for any changes indicating an interaction.

If your horse is showing evidence of a drug or medication interaction, immediately call your veterinarian. If the interaction is severe or life-threatening, and you cannot reach your veterinarian, call the National Animal Poison Control Center at 888-426-4435.

The National Animal Poison Control Center is a division of The American Society for Prevention of Cruelty to Animals and is staffed 24/7, 365 days a year by veterinary toxicologists. The center charges a fee for these services that may be applied to a credit card, but it is a small price to pay when the life of your horse is involved.