How do monthly heartworm preventives work? Whether the preventive you choose is given as a pill, a spot-on topical medication or as an injection, all approved heartworm medications work by eliminating the immature (larval) stages of the heartworm parasite.

Do dogs really need heartworm pills?

Heartworm disease can be prevented in dogs and cats by giving them medication once a month that also controls various internal and external parasites. Heartworm infections are diagnosed in about 250,000 dogs each year. 1 But there is no good reason for dogs to receive preventives all year; it is just not needed.

How quickly does Heartgard work?

How quickly does Heartgard Plus work? Heartgard Plus starts acting as soon as it is swallowed, absorbed and enters the dog's circulation.