NASHVILLE, Tenn -- A Channel 4 I-Team investigation reveals more than 24,000 complaints in 2008 about animals getting sick from a medication designed to keep them healthy. One hundred-ten pets died that year after using medicine sold by Sergeant's Pet Care Products, according to the Environmental Protection Agency.

Two Middle Tennessee pet owners said they used products made by Sergeant's, and not only did their animals get sick, one of them died.

"It made me sick. I thought to myself, 'I'm trying to help my animals, and I've killed one of them,'" said pet owner Tammy Nauman.

But Caryn Stichler, vice president of marketing for Sergeant's, said if the company's products are used properly, there should be no problems.

"We feel very comfortable that our products are safe and effective," Stichler said.

The I-Team began investigating Sergeant's medication after several Middle Tennessee pet owners contacted the I-Team about how their animals had suffered after using the medicine.

Nauman said she put the flea and tick medicine, made specifically for cats, on four of her cats. All four started twitching and scratching.

One of her cats, Hal, died three days later.

Melissa Harris put the flea and tick medicine, made by the same company, on all five of her dogs. She said all five quickly started shaking and crying.

"To see each one of them with the symptoms was heartbreaking," Harris said.

Both Nauman and Harris said they had no idea the EPA was investigating all types of topical flea and tick medications, the type of product applied directly to the skin.

In the EPA's findings in 2008, there were 44,000 complaints of pets having complications from topical medicine, and 600 pets died. The EPA told the I-Team half of those complaints they received were about Sergeants products, and 110 of the deaths were reported after pet owners put Sergeant's products on their animals.

"Sergeant's, if you look at us as a manufacturer, had a large percentage of our adverse effects, 99 percent, were minor, which is a higher percentage than the industry average," said Stichler.

But Goodlettsville veterinarian Mary Ergen said she tells her customers not to use any low-cost topical flea and tick medication from any company.

"People need to know there's that toxicity problem with these medicines, and it can harm your pet," Ergen said.

So why do so many people use these products?

Pet owners said the cost of Sergeant's and other topical medications are much cheaper than Frontline, the topical medication most veterinarians recommend.

The difference between the products, according to the EPA and Sergeant's, is that Frontline uses different chemicals.

But Stichler said the problem arises when people misuse their products, such as putting the cat product on a dog or not using the correct dosage.

Nauman and Harris said they did use the medication properly.

Ergen treated Nauman's cats.

"She does exactly like she's told, applies it the way she's been instructed to apply it, and it kills her cat," Ergen said.

In a letter to Nauman sent from Sergeant's, the company suggested her cats licked the product off each other. Inside the box of the medication, there is a warning to keep animals away from each other for 24 hours after application.

Nauman said her cats didn't groom each other, and Harris said her dogs started having problems within minutes of applications.

Harris said only her German shepherd, Wootee, groomed one of her pekapoms, and he did get sick.

"(It was) heartbreaking. These dogs are our children," Harris said.

An EPA representative specifically acknowledged in an e-mail to the I-Team that some of the 44,000 complaints turned up problems that initial product testing never detected before the product was licensed.

"Sergeant's tests its products well beyond what is required by the EPA," Stichler said. "There will always be, because this is a chemical, some adverse events, just as people are allergic to milk, or peanut butter, or even chemicals."

But Ergen said until further testing is completed, pet owners should be cautious.

"I wouldn't put it (low-cost topical medication) on my own animal. There are safer products out there," Ergen said.

Stichler also told the I-Team there have been millions of satisfied customers, and the adverse reactions are reported in only about 1 percent of total sales.

The EPA said it is now working with all the markers of topical medication to possibly change their labels to make them clearer and easier to understand.