The U.S. Food and Drug Administration (FDA) is warning pet owners that dogs and other animals may die if exposed to a cream used to treat and prevent skin cancer.

Five cases of dogs that became ill and died after accidentally ingesting a cream called Fluorouracil Cream USP 5% (5-FU) have been reported to the FDA.

“People using this medication should use care when applying and storing the medication if they are also in a household with pets, as even very small amounts could be dangerous to these animals,” the FDA said in the Jan. 18 warning.

Also marketed under the brand names Carac, Effudex and Fluoroplex, the topical cream is prescribed to treat precancerous sun-damaged skin and some basal cell skin cancers. Fluorouracil is part of a class of drugs called antimetabolites and prevents the growth of abnormal skin cells.

Although the FDA received five reports of dogs dying after ingesting the cream, it’s possible more have gone unreported.

In one case reported to the FDA, two dogs were playing with a tube of the cream. One of the dogs punctured the tube and began vomiting and seizing within two hours. Twelve hours later, the dog was dead.

“In a separate case, a dog located his owner’s tube of Fluorouracil and ingested its contents,” the FDA wrote. “The owner realized the dog had ingested the medication and rushed him to the veterinarian. The veterinarian attempted treatment, but the dog’s condition declined over three days and he was ultimately euthanized.”

While the side effects on dogs were reported by the American Society for the Prevention of Cruelty to Animals (ASPCA) in April 2001, the knowledge that Fluorouracil could kill dogs if ingested was not widely dispensed.

“Pharmacists and other medical personnel may be unaware of the severe problems that arise when dogs ingest 5-fluorouracil,” the ASPCA wrote at the time. “Occasionally, owners are erroneously told that ingesting 5-fluorouracil is harmless or that dogs may develop only mild oral and gastrointestinal irritation.”

In its toxicology brief, the ASPCA reported that a dog weighing less than 70 pounds could die from ingesting half a tube of the cream.

While dogs are the main focus of the FDA warning, Fluorouracil may also have an effect on other pets.

“Although the FDA has not to date received any reports involving cats, they are also expected to be extremely sensitive to Fluorouracil cream,” the FDA said. “If an owner applies Fluorouracil cream to an afflicted area and touches their cat, the cat may accidentally ingest the medication when grooming itself and suffer adverse events.”

The FDA recommends that people take precautions to prevent their pets from accidentally ingesting the drug, such as storing all medications out of reach and avoiding leaving any residue of the medication on hands or carpeting.

Health care providers should advise patients about the risk to pets, and veterinarians who encounter dogs exhibiting certain symptoms should ask whether anyone in the household uses Fluorouracil.