If you take a multivitamin every morning and perhaps a supplement or two because you care about your health, does it make sense to do the same with your dog or cat? Stephanie Pendleton of Emerald Isle, North Carolina, would say yes.

Pendleton caught on to pet supplements a few years ago when she noticed that her 13-year-old cat, Sierra, wasn't up to her usual antics.

"She was having a hard time jumping up on the counter, and she wasn't playing as much as she used to," Pendleton says. "Finally, she just spent less time up there, I think, because it was painful for her."

Pendleton researched Sierra's problem online, and learned about the joint supplements glucosamine and chondroitin. She asked her veterinarian, and they agreed to give the cat a product that combines both joint supplements.

"Sierra is jumping all over the place again," Pendleton says.

Now, Pendleton is a believer in pet supplements. She gives Sierra and her other cat, 2-year-old Serenity, a multivitamin, probiotics to help their immune systems and essential fatty acids for skin and coat health. Sierra gets seven pills, Serenity four.

Demand for supplements is on the rise. The pet supplement market has grown about 15 percent annually since 2000 and is now a $1.3 billion business, according to the National Animal Supplement Council. Simmons Market Research Bureau says approximately 17 percent of pet owners give their cats and dogs some type of supplement.

A pet supplement is a product that is intended to complement the diet and help support and maintain a normal biological function. Products range from multivitamins for overall health to targeted formulas that claim to alleviate joint problems or canine cognitive dysfunction.

Dr. Tim Montague, a veterinarian at Eads Animal Hospital in Eads, Tennessee, started using supplements in 1992. He was wary at first because he didn't learn about them in veterinary school, and there weren't many on the market. But when an old professor of his recommended a joint supplement for one of Montague's patients, he took notice. Montague's golden retriever Ayla had an arthritic shoulder, so he also tried a joint supplement on her.

"She could barely make it up and down the stairs, but within a week after the supplement she was running and catching Frisbees in the yard," Montague says. "That sold me on that product." He said his patients have had good success with joint supplements and he prescribes them all the time.

Dr. John Bauer, professor of clinical nutrition at Texas A&M University's College of Veterinary Medicine, offers four factors to consider when evaluating supplements:

1. Product quality. How much of the functional, active ingredient is in the product? Responsible manufacturers will have 800 numbers on the package to call with technical questions about the ingredients, and your veterinarian should know what specific questions to ask.

2. Efficacy. Is there any scientific basis to support the use of this supplement? If information about product testing isn't available on the company's Web site or elsewhere, call the company for details about the studies that have been performed.

3. Tolerance. Check the list of ingredients carefully before giving a supplement to your pet. For example, a supplement might include lactose, which some cats and dogs can't tolerate. It's a good idea to consult your veterinarian first to discuss how the supplements may react with any medications the pet is taking.

4. Safety. A product's safety should be proven. For example, the company might state in its literature that it was tested in high doses on mice and found to be safe, or the number of adverse events reported might be few to none.