There are many products available for heartworm prevention. The best option is one that suits your lifestyle and is the most convenient and thereby safest for your pet.
A 'Once a year heartworm injection' known as Proheart is a very convenient method of protection. Proheart is administered by your vet and will remove the risk of you forgetting daily or monthly dosing. Monthly heartworm preventatives and treatments include spot on treatments and chewables which also incorporate other forms of parasite protection for fleas and worms.