We currently offer two forms of heartworm preventative, Heartgard Plus (ivermectin/pyrantel) and Trifexis (milbemycin oxime/spinosad). Both are administered once a month (every 30 days). Heartgard Plus is a beef flavored chewable. It kills the infective stage of the heartworm along with roundworms and hookworms, the most common intestinal parasites of dogs. Trifexis is a beef flavored tablet. It also kills the infective stage of the heartworm, roundworms, and hookworms, as well as whipworms and adult fleas. It does not kill ticks. The most common side effect of Trifexis is vomiting, but administration on a full stomach will prevent this. Trifexis should not be administered to dogs with a history of seizures.