Heart worm is a disease spread by mosquitoes. Your dog could be at risk if you live in an infested area. Your veterinarian may recommend a monthly preventative heart worm medication, administered regularly until your vet tells you that it’s safe to stop for the winter. If you forget to give your dog his heart worm medication one month, call your vet.  He’ll likely recommend that you give your dog his pill as soon as you’ve recognized that you’ve missed it and then continue as normal next month. Usually missing one dose is not a problem.  If you forget for two consecutive months, your dog may be susceptible to heart worm. As ask your veterinarian to check him out.