Will Cimetidine Hurt the Foal?

Q.  I have a gray mare that's 19 years old and has melanomas under her tail. She's been on cimetidine for a couple of years to treat them, which has been working fine. I cut off her cimetidine before breeding her recently (she was just checked in foal at Day 20), and the melanomas haven't enlarged much since then. I would like to put her back on the cimetidine to keep the melanomas under control, but don't want to compromise her pregnancy. One veterinarian I spoke with says that cimetidine crosses the placenta (passes from the mare to the fetus) and I shouldn't use it, while another says it would be OK. Who's right?

Jason Brown, Winston-Salem, N.C.

A. We have no information in the mare on whether or not cimetidine crosses the placenta. The evidence that the drug works at all is controversial. Melanomas might increase in size during pregnancy because of the pregnancy-induced variations in the immune response, and then again they might not. I would not worry about putting the mare back on cimetidine, however, as I am a skeptic on whether cimetidine works at all. I likely would not have even used the drug.

About the Author

Michelle LeBlanc, DVM, Dipl. ACT

Michelle LeBlanc, DVM, Dipl. ACT, is a theriogenologist (reproduction specialist) for Rood and Riddle Equine Hospital in Lexington, Ky. She was previously a professor in equine reproduction at the University of Florida. Her interests deal with mare infertility, embryo transfer, placental infections in mares, and acupuncture in infertile mares. She owns Thoroughbred/warmblood crosses and she competes in dressage.

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