They're sold over the counter, on the Internet, and in tack shops, so herbal supplements must be safe for your horse, right? Not necessarily, report researchers from Norway and China.

According to this group of scientists, some commonly used herbs, when applied to cultured liver cells at biologically relevant doses, are able to activate specific liver enzymes, particularly those belonging to the cytochrome P450 (CYP) family of enzymes. These enzymes play a major role in drug metabolism, so the concern regarding herbs that induce CYP enzymes (or other liver enzymes) is the potential for the occurrence of drug-herb interactions.

In the study, "Trade herbal products and induction of CYP2C19 and CYP2E1 in cultured human hepatocytes," both St. John's wort (Hypericum perforatum) and Ginkgo biloba induced CYP enzymes. The researchers concluded that both herbs "should be considered possible candidates for clinically relevant drug-herb interactions."

While this particular study was conducted using human hepatocytes (liver cells) in culture, these findings are of concern to horses and their owners, according to Carey Williams, PhD, Equine Extension Specialist and assistant director of extension at the Equine Science Center at Rutgers, The State University of New Jersey