Liquid flea and tick treatments for dogs and cats have been on the market for more than a decade. But following a recent increase in reports of adverse reactions among pets, the Environmental Protection Agency has intensified its scrutiny of the products, warning pet owners that the treatments may have serious, even fatal, side effects.

Spot-on flea and tick treatments are applied topically, usually between the animal’s shoulder blades or along the spine. There have been no recalls of the products, and the agency said in May that there is no reason to avoid them.

But the E.P.A. is investigating a large number of anecdotal reports involving both cats and dogs who received spot-on treatments and suffered problems like skin irritations, hair loss and tremors, according to agency spokesman Dale Kemery. The initial reports were not verified by veterinarians.

The agency now is evaluating all available data on the pesticides, including reports of adverse reactions, the clarity of the directions and label warnings, and the pre-market safety data submitted in support of the products.