This depends on what type of heartworm pills you choose to give him. I recommend that even with over-the-counter products you ask your vet before administering medication, since only he will know your pet’s full medical history. The most common heartworm preventatives are Macrocyclic Lactones. These are normally administered as a pill on a monthly basis (although topical applications are available). The main types of oral ML available are Ivermectin (eg Heartgard?, Iverhart Plus) and Milbemycin (e.g. Interceptor,Sentinel). These drugs are parasiticides, and will prevent (and treat) heartworm infestation. There are also daily treatments such as Diethylcarbamazine but these tend to be less popular.