By Victoria Gill
Science reporter, BBC News
Vultures have disappeared from large swathes of the skies in India
A veterinary pain drug can be lethal to vultures that eat the carcasses of treated livestock, say scientists.
Ketoprofen is an anti-inflammatory that is used in India to treat cattle.
It had been proposed as a replacement for diclofenac, which scientists say brought some species of Asian vulture to the brink of extinction.
A study published in the Royal Society journal Biology Letters says it causes the birds to suffer acute kidney failure within days of exposure.
This is the same toxic effect caused when vultures feed on the carcasses of animals treated with diclofenac.
Researchers had thought that ketoprofen would be less harmful because it metabolised faster by cows, and converted within hours into a form that is not dangerous to vultures.
But an international team of scientists that carried out safety tests on the drug, found that doses administered to cattle in India were sufficient to kill the birds.
Richard Cuthbert from the Royal Society for the Protection of Birds (RSPB) led the study, which involved researchers from academic institutes and conservation organisations in Europe and Africa.
Some vultures are poisoned when they feed on the carcasses of recently treated livestock
These included the Bombay Natural History Society, Namibia's Rare and Endangered Species Trust and the University of Pretoria in South Africa.
His team carried out tests on the more common species of vultures, using them a surrogate for endangered Asian vultures.
Their tests showed that meat from animals that had been treated with ketoprofen could be lethal for the birds.
"We also collected samples [of tissue] from cattle carcasses all over India, and analysed them looking for diclofenac and other related drugs," he told BBC News.
"We found carcasses with ketoprofen within them at levels that are likely to cause toxicity."
"It's fair to say that ketoprofen is less toxic than diclofenac," said Dr Cuthbert, "and if it's used properly, there probably would be a very low risk to vultures.
"But we know that these drugs are often not used properly, and two or three times the dose is often administered to cattle."
Rhys Green, a zoologist from the University of Cambridge who was not involved in this study, said the findings were important.
"This reveals that a veterinary drug that some pharmaceutical companies in the Indian subcontinent would like to sell more of is not safe for vultures, which have already been reduced to very low levels by diclofenac."
"Ketoprofen isn't a big problem for vultures at the moment because little is used. But it would hamper efforts to restore vulture populations if its level of use increased to rival that of diclofenac."
The RSPB is promoting the use of what it says is now the only safe alternative to diclofenac - a drug called meloxicam.
"We'd like to know of more safe alternatives... and we're asking the Indian pharmacautical industry to step up and test them," said Dr Cuthbert.
The Indian government has banned the production of veterinary diclofenac, but Dr Cuthbert said that there was still a problem with vets using human diclofenac in cattle.
"It's still cheaper than meloxicam," he told BBC News. "But the price of meloxicam is coming down... as more companies produce it."
Professor Green concluded: "There are also other drugs of the same family in use, which have not been tested for their effects on vultures.
"Testing is expensive and pharmaceutical companies aren't required to do it."