Dog owners are being urged to check with their veterinarians about a heartworm medication that has been recalled.

Proheart 6 is a twice-a-year, time-released heartworm medication that has been used to treat millions of animals.

On Friday, the Food and Drug Administration recalled the medicine after thousands of dogs got sick.

The main ingredient is Moxidectin. It has been used in horses and cattle without any problems. But once ProHeart 6 was used to treat dogs, health questions started to pop up.

The FDA said that about 500 dogs have died, but it is not clear if those deaths can be blamed on the medicine.

Dogs of different breeds and sizes have suffered from sudden lethargy, uncontrolled bleeding, vomiting, diarrhea, heart and liver problems and seizures.

The FDA said that, at this point, it doesn't understand why the product is causing the problems since no problems have been reported with other animals or even with dogs during the product trials.