Arthritis supplements bought by millions of pet owners for their dogs, cats and horses sometimes skimp on the ingredients the makers claim can help aching paws and aging joints, and some contain high amounts of lead, an independent laboratory found.

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.

“You can’t ask a dog or a cat to give you a subjective impression of how they’re feeling after taking the product for several days. They can’t say, ‘On a scale of 1 to 5, I feel better or worse,'” Blumenthal said.

Giving supplements to an ailing pet can make its owner feel better, though. “The owner shelled out money for the pills and wants to believe they are helping,” Blumenthal said.

Up to one-third of dogs and cats in the U.S. are given supplements, a government report estimates. Sales of pet supplements have roughly doubled since 2003, to nearly $1 billion a year in the United States, according to the Nutrition Business Journal. These supplements are sold over the Internet and at pet supply stores and some groceries.

Many pet owners believe they make a difference.

Nicole Albino, who lives in New York City, said her dog Chakka was constantly chewing and licking his knees until her veterinarian recommended glucosamine and chondroitin.

After taking the pills for a year, “he’s definitely been licking his knees a lot less,” she said. The dog resumed when she ran out of the stuff for a few weeks. “It just seems to help,” Albino said.

Few high-quality studies have tested the effectiveness of animal supplements. The Food and Drug Administration says these products are not bound by quality rules for human ones.