Four of the six joint supplements for animals tested by ConsumerLab.com lacked the amounts of glucosamine or chondroitin promised on their labels or had other flaws, such as lead. Wider testing by a trade group of 87 brands found that one-quarter fell short.

Over-the-counter dietary supplements for humans do not have to be proven safe or effective before they are sold, and pills for pets get even less scrutiny.

"There is and there always has been" a quality problem, although many companies do a good job, said Mark Blumenthal of the American Botanical Council, which tracks research on herbal products.

Even when these supplements contain what they claim, there is little evidence that they work, veterinary experts say. A large government study of people with arthritis found that glucosamine and chondroitin did no better than dummy pills in easing mild pain. Testing these supplements on pets is more difficult.