Unfortunately, this is the case. Immiticide, the only FDA approved drug used to treat active heartworm infections, is currently unavailable. The supply has been depleted because the manufacturer is experiencing issues with production. It is uncertain how long this shortage will last. Immiticide should not be confused with heartworm preventatives. It is a powerful adulticide that is given by injection to treat dogs that already suffer from heartworm disease. Heartworm preventatives are medications used to prevent heartworm infections. Because of this shortage, it is of utmost importance to be diligent about administering heartworm prevention to your dog. This shortage emphasizes the importance of routine annual heartworm testing for all dogs over 6 months of age or whenever dogs have missed doses of monthly prevention. If heartworm prevention is not given once monthly, the risk of heartworm infection increases and this includes missing only a single monthly dose. Likewise, if an inadequate dose of heartworm prevention is administered, the risk of heartworm infection increases. Heartworm preventatives cannot be broken in pieces in order to treat multiple dogs at a lower cost. This is because the active ingredient is not evenly dispersed through the tablet/chewable so there is great risk of under-dosing.