Experts in horse racing are questioning whether a relatively new class of drugs could be the culprit behind an uptick in the number of horse deaths in California and Kentucky. The drug, an obscure treatment for osteoporosis, could be masking vulnerability in bones for younger horses. Almost all the deaths in Kentucky and California have been triggered by musculoskeletal issues. At least 23 horses have died at the Santa Anita Park in California in the past three months, and horse fatalities at Kentucky’s tracks nearly doubled in 2018.

Bisphosphonates are osteoporosis medications that were approved about five years ago for use in horses four or older, but vets can prescribe it legally for other bone problems in younger horses. The drug kills off the cells, called osteoclasts, that clear away bone with microdamage. “We’re concerned about the science that shows when it is given to young mammals it can cause stress fractures,” said Ed Martin, president and CEO of the Association of Racing Commissioners International. “We don’t have equine-specific research… but other research shows its connection to stress fractures and the link to catastrophic injury.”