Q:My wife and I were told that giving Bute (phenylbutazone) to a mare in foal can cause heart defects in the foal, similar to aspirin taken by women in early pregnancy. Is there any connection?

Tom

A:Signs of phenylbutazone toxicity in foals whose dams are treated with the drug are rare. However, some researchers have reported that the blood levels in foals from treated mares are similar, indicating placental transfer. Also, the clearance of Bute in the foals was much slower than in their dams (17 hours in foals vs. four to six hours in the dams). Since Bute toxicity in newborn or very young foals is easily induced, it would be prudent to limit the use of this drug in pregnant mares.