Diclofenac, which was developed to treat arthritis and other painful health conditions, is restricted to human use in America, but that’s not the case elsewhere. Livestock farmers across Europe, Asia, and Africa have routinely used diclofenac to treat fever and immobility in cattle, sheep, and goats because it’s cheap and widely available. These farmers tend to overlook the drug’s danger to vultures because it’s such a boon for business, preventing the loss of their precious livestock to lameness and infection.

Vultures ingest diclofenac when they feed on carcasses of deceased livestock that have been treated with the drug. “It causes kidney failure,” says José Pedro Tavares, director of the Zurich-based Vulture Conservation Foundation, one of several vulture advocacy groups that has cropped up since the early 2000s. “Vultures die within two days with a very small dose.”

Farmers have used veterinary diclofenac since the early 1990s. At the end of that decade, Vibhu Prakash of the Bombay Natural History Society reported an unusually steep decline in Asian vulture populations in India and Pakistan. But the cause of the decline—diclofenac poisoning—wasn’t uncovered until early 2000s. In 2006, India, Nepal, and Pakistan banned the use of veterinary diclofenac after those countries saw their vulture populations take a nosedive. Bangladesh, concerned about its own dwindling vulture population, also implemented a diclofenac ban in 2010. Across Asia, one species, the oriental white-backed vulture, declined to just one-thousandth of its pre-1990 population by the mid-2000s as a result of diclofenac poisoning. Currently the drug is still approved for veterinary use in Europe, yet vulture-conservation groups like Tavares’ are pushing for a ban.

In Europe and parts of Asia, farmers have easy access to a different non-steroidal anti-inflammatory drug called meloxicam that’s as effective as diclofenac but not toxic to vultures, says Tavares. It hasn’t been widely adopted, however, because even though it ultimately alleviates animals’ pain, it also appears to cause pain when it’s first injected. In a 2014 study on goats, the veterinary meloxicam formulas used had a higher pH and drug concentration, which can cause a sharp stinging sensation. Agitated livestock aren’t easy to inoculate, and may try to kick or run when they are in pain.

Looking to avoid injury and save time, farmers often turn to two other NSAIDs, ketoprofen and aceclofenac, which appear to hurt livestock less. But, like diclofenac, recent studies reveal these drugs—which are chemically similar to diclofenac—also appear deadly to vultures.

“Right now, getting all veterinary drugs, at least the NSAIDs, safety-tested on vultures ahead of being licensed … is the single highest priority for vulture conservation,” says Chris Bowden, the program manager of Saving Asia’s Vultures from Extinction.