Trifexis Flea Heartworm Chewable For Dogs. Trifexis is an exciting new flea and heartworm control chewable. It combines two popular ingredients - spinosad, a flea control ingredient also found in the comfortis flea pill, and milbemycin oxime, the heartworm preventative found in sentinel. Trifexis for dogs also controls adult hookworm, roundworm and whipworm infections - but it does not control ticks. Trifexis requires a prescription because it contains a heartworm preventative, which like all heartworm medication requires an annual check up and prescription from your veterinarian.