Q: A mare I am planning to breed had shoulder surgery almost two years ago (part of her scapula was removed due to a fracture). Although she healed well, she recently slipped and reinjured the shoulder. She’s on Bute twice a day. I have read that Banamine is safer for pregnant mares than Bute. Should I consult with my veterinarian about switching her to Banamine instead of Bute?

—Brenda, via email

A: This is a very good question, and amazingly, we don’t have a really good answer. Despite being around for years, phenylbutazone (Bute) has never been fully investigated in pregnant or lactating mares. There is a lot of research regarding its toxicity in the gastrointestinal tract and the kidneys. Because of these toxicities, the only reasons for using Bute are because it’s cheap, and it’s effective for musculoskeletal pain. There are a couple of reasons that Bute is the most toxic of the non-steroidal anti-inflammatory drugs (NSAIDs).

All of the NSAIDs are metabolized by the liver before being eliminated. But with Bute, giving high or repeated doses saturates the enzyme system that the liver uses to get rid of the drug. Once the system is saturated, the Bute starts accumulating in the horse and the blood concentrations of the drug become very high and unpredictable. So, it takes longer than 24 hours to get rid of a whole dose