Authorities and pet owners are beginning to raise serious questions about the safety and effectiveness of animal medicines, mirroring worries over human drugs like the painkiller Vioxx.

Tested on just a couple hundred animals, a drug meant for pets is less apt than a human one to show all its failings until it reaches market, veterinarians say. More than 700 drugs have been approved for pets, but many others are used legally without explicit approval for animals. Most pet drugs were first developed for people.

But there is deepening awareness that what works in people may not work in animals. Indeed, each species of animal — even varying breeds — may react differently to the same drug.

Further, animals can't say if a drug makes them feel bad. "I can't tell until you see something physical," says Laurryn Simpson of Commerce Township, Mich., who founded the Web site dogsadversereactions.com.

The worries arise at a time when intensifying demand has pressured the FDA to hire more reviewers and sort through research more quickly to decide whether to approve new pet drugs. Given the smaller pet market, many companies save development costs by relying on cheaper experiments with typically a tenth as many subjects as in human tests.

Dr. Stephen Sundlof, the vet who directs the FDA's Center for Veterinary Medicine, says if the agency insisted on the same size studies as for people, "we would have very few drugs" with formal approval for pets. But he adds, "The rigor is every bit as great as with human drugs."

Since the year 2000, reports of side effects in animal drugs have gone up about 90 percent, to 34,603 last year, FDA records show. The agency ties the growth to new types of drugs and greater understanding of potential dangers — not worsening safety. However, vets say that the vast majority of side effects are never reported, so it's hard to gauge overall safety.

And many vets barely speak of possible side effects when they recommend a drug, some clients complain.

Jean Townsend, of Johns Island, S.C., says her vet breathed "not one word" of side effects when he prescribed the painkiller Rimadyl for her arthritic, limping Labrador retriever.

Encouraged by an advertisement showing dogs romping playfully, Townsend says she was glad to soothe her pet's aches. Within a month, though, he collapsed and began to vomit blood. A week later, he had to be put to sleep, his kidneys and liver ravaged beyond repair, his medical records show.

"The medicine blew him apart," says Townsend. Her vet and FDA reviewers all blamed the drug, which was originally targeted for humans.

Without admitting wrongdoing, drug maker Pfizer paid out roughly $1,000 to each of 300 pet owners, including Townsend, to settle a lawsuit in 2004.

Rimadyl, taken by more than 10 million dogs since 1997, is now tied to more than 3,000 pet deaths, FDA data show. Many of these pets, predominantly dogs, had damaged livers or kidneys.

Rimadyl is in the same broad NSAID family of drugs as Vioxx. However, unlike Vioxx, it stayed on the market.

The FDA stresses the need for medicines like Rimadyl, partly because pets cannot tolerate the range of pain-killing alternatives that humans can. Dogs are more sensitive to aspirin than humans. And a single Tylenol can kill a cat.

But sometimes a drug's risk is too great to accept. A heartworm medicine, Proheart 6, was pulled from the market in 2004 after FDA researchers found evidence of fatal side effects.