: Giving preventative medication to a pet that already has adult heartworms can cause a sudden massive die-off of larvae. This can trigger a severe, shock-like reaction that may be fatal .

: A veterinarian ensures the preventative won't have dangerous interactions with other medications your pet may be taking.

In the United States, you . The U.S. Food and Drug Administration (FDA) regulates all effective heartworm preventatives as prescription-only medications. Why a Prescription is Required