Nuheart protects against heartworm, which is a deadly parasite that is spread by mosquitoes. When bitten by an infected mosquito, heartworm larvae are transmitted into your dogs bloodstream and swim to the chambers of the heart, where they become adults. Eventually heart failure ensues, which can be deadly if untreated. Treatment for heartworm is extremely difficult and dangerous, so prevention is key. This product is safe for puppies from 6 weeks of age. Be careful to never overdose in any dog, particularly border collies. If you have missed a heartworm prevention dose, always check with your vet that prevention is safe to give as testing may be required.