Pills or chews that protect dogs and cats against fleas can cause neurological problems such as stumbling or seizures, and pet owners need more warning, the Food and Drug Administration said Thursday.

The FDA issued an alert to owners and veterinarians and said it was requiring clearer labels on the products.

The flea pills and chews all contain a pesticide called isoxazoline, the FDA said. They include products sold under the brand names Bravecto, Nexgard and Simparica.

“Another product in this class, Credelio, recently received FDA approval. These products are approved for the treatment and prevention of flea infestations, and the treatment and control of tick infestations,” the FDA said in a statement.

“Isoxazoline products have been associated with neurologic adverse reactions, including muscle tremors, ataxia and seizures in some dogs and cats,” the FDA added. Ataxia is a lack of muscle control and can cause animals to stumble or twitch.

“Although most dogs and cats haven’t had neurologic adverse reactions, seizures may occur in animals without a prior history,” the FDA added.

The labels on the products, sold mostly as flavored chews, already mention the risk of neurological side effects, and the risks have been reported by consumer groups. It is just important for vets and owners to get more information, the FDA said.

“The FDA carefully reviewed studies and other data on Bravecto, Credelio, Nexgard and Simparica prior to approval, and these products continue to be safe and effective for the majority of animals,” the agency said.

“The agency is asking the manufacturers to make the changes to the product labeling in order to provide veterinarians and pet owners with the information they need to make treatment decisions for each pet on an individual basis.”

There are good reasons to use flea products.

Fleas and ticks carry a range of diseases, including plague and Lyme disease, which can affect animals and their owners. Pets can develop allergies or sensitivities to the bites, and lose large patches of fur as a result. They can also become susceptible to worms and other parasites if they suffer allergic reactions to flea bites.

The FDA said hundreds of pesticides are on the market to protect against fleas and ticks. They include “spot-on” products applied to an animal’s skin, flea sprays, powders and collars. Many can cause nausea, vomiting and neurological side effects.

It is important to make sure that the right product is used, the FDA said.

“Flea treatments meant for dogs can be deadly if given to cats instead. It is important to use only flea and tick products specifically designed for cats, and to administer the proper dosage,” the FDA cautions.

Products containing permethrin are safe for dogs but can be toxic to cats.

“If you have both dogs and cats in your household, you should be aware that using a permethrin ‘spot-on’ product on a dog may cause illness in a household cat,” the FDA noted.

The FDA and the Environmental Protection Agency have warned about potential dangers of spot-on flea products and required makers to include clearer labels.

“If your pet experiences a bad reaction from a spot-on product, immediately bathe the pet with mild soap, rinse with large amounts of water, and call your veterinarian,” the FDA advises.

The isoxazoline products are designed to interfere with the insect nervous system. They were not believed to interfere as much with the nervous systems of mammals. Earlier this year, one group of researchers proposed testing them in people to protect against mosquito and other insect bites.