Missing a month of Trifexis may leave your dog unprotected against fleas, heartworms, and other parasites. As a monthly preventative, Trifexis works by killing any parasites that may be present on the dog's skin and fur or in the dog's bloodstream. If a dose is missed, it can take longer to kill existing parasites and may reduce the effectiveness of the medication overall. It is important to administer the medication on schedule, as directed by your veterinarian. If you miss a dose, it is best to give it as soon as possible and then return to the regular schedule. If you are unsure of what to do, it is best to consult with your veterinarian for guidance.