Heartgard Plus and Sentinel are two popular heartworm preventatives for dogs. Both are flavored chewables given once a month that kill parasites like heartworms, roundworms and hookworms. However, Sentinel is also effective against fleas, flea eggs, and whipworms. While Sentinel can be used for dogs as young as 8 weeks, Heartgard Plus is recommended for dogs 6 weeks and older. Sentinel tends to be more expensive but provides additional protection against fleas and whipworms. For collie breeds, Heartgard Plus may cause fewer adverse reactions due to differences in active ingredients between the two medications.