It depends on a number of factors, including where you live, the climate, and your dog's activity level. However, in general, it is recommended that dogs receive heartworm medication year-round, even in the winter. This is because the mosquitoes that transmit heartworms can survive in warmer climates year-round, and even in cooler climates, they may be active during periods of warmer weather. If you are unsure whether or not your dog needs heartworm medication, speak to your veterinarian.