Rifampin is commonly used off label to treat infections in horses, and occasionally in dogs and cats. It is usually given by mouth in the form of a capsule or compounded liquid. The most common side effects are vomiting, diarrhea, or reduced appetite. Liver dysfunction is also possible. Use with caution in pets that have pre-existing liver disease, are pregnant, are elderly, or are lactating. If a negative reaction occurs, please call your veterinary office.