Reports of pets experiencing medication side effects are on the rise, and concerned owners are working to raise awareness about the problem.

One drug was even removed from sale last year after several thousand complaints to the federal Food and Drug Administration.

Pet pharmaceuticals are big business. The Animal Health Institute, representing manufacturers, reports that in 2003 American consumers spent about $2.5 billion on health products for companion animals (dogs, cats and horses), including medicine and other items. Nearly 300 medicines are approved by the FDA for use in companion animals.

The FDA's Center for Veterinary Medicine in Rockville, Md., monitors reports on the safety and efficacy of animal drugs and medicated feeds. Last year, the center received 28,424 reports of side effects or ineffectiveness from drug companies, veterinarians and animal owners. That's an increase of more than 5,000 over 2003, said Rae Jones, an FDA spokeswoman.

Jones said that while the center cannot determine the exact cause for the rise, increased awareness of adverse drug events is probably a "major contributing factor." Also, "new products and therapies continually enter the marketplace, which have associated risks with their usage," she added.

Entering the marketplace in June 2001 was ProHeart 6, a six-month injection against heartworm. Heartworm disease is a potentially fatal condition caused by parasitic worms. Many dogs take a preventative.

Laurryn Simpson had always given her pug, Velvet, a chewable monthly preventative. But in March 2003 Simpson's veterinarian was out of that and recommended ProHeart 6.

Back then Simpson was driving a truck and needed to get on the road. Although she was reluctant to try a different drug, she went ahead with the injection.

"About 10 days later Velvet had vomiting and diarrhea and wouldn't eat," said Simpson, of Commerce Township, Mich. "A week after that, the seizures started." They still occur intermittently.

Simpson scoured the Internet and discovered that many other dogs that had received the injection experienced similar complications and worse, including death.

The site provides updated information on drugs, vaccines and pet reactions. Between 600 and 700 readers also subscribe to Simpson's monthly newsletter.

ProHeart 6 is no longer available. At the request of the FDA — after the Center for Veterinary Medicine received nearly 6,000 reports of adverse effects from the drug — its manufacturer, Fort Dodge Animal Health, voluntarily ceased production in September.

That month, Simpson said, her Web site logged 313,241 hits.

Cheryl Hartman of Pasadena, Texas, is a frequent visitor to Simpson's site. Her Doberman, Silk, died in June of heart arrhythmia that Hartman links to the drug Deramaxx.

Hartman said her veterinarian prescribed the nonsteroidal anti-inflammatory for a leg injury.

After two doses, Silk was hospitalized for six days with internal bleeding, costing Hartman more than $4,500 and causing what she calls "major permanent damage" to the Doberman's health.

Novartis Animal Health US, which supplies Deramaxx to vets, cautions about potential side effects at www.deramaxx.com.

Joseph Burkett, a Novartis spokesman, said a client information sheet is packaged with each product shipment. He added that communicating with veterinarians about proper use of drugs is "a top priority" for Novartis.

"In addition, it's always important for pet owners to ask their veterinarian for information about any medication their pet may be taking," Burkett said. "Because the client sees the pet every day, they're in the best position to detect changes in their condition."

Hartman said her vet never offered an information sheet, and assured her that the drug was safe after she specifically asked.

Katrina Mealey, director of the Veterinary Clinical Pharmacology Laboratory at Washington State University in Pullman, said owners can work to minimize the risk of drug side effects.

First, be especially cautious with senior pets.

"Older animals in general are more likely to experience adverse drug reactions because their kidney and liver function [to detoxify and excrete the drug] may be compromised," she said.

Also, realize certain breeds may be sensitive to specific medications.

"We know from my own research that many herding breed dogs [collies, shelties, Australian shepherds] have a genetic mutation that predisposes them to neurotoxicity if given one of several drugs," Mealey said. More information is at www.vetmed.wsu.edu/vcpl.

And remember it's not just prescription drugs that hold potential dangers, she said. "Over-the-counter herbal products and so-called nutriceuticals are also not 100 percent safe."