After a year's absence, Sentinel has returned to the market and is once again available for heartworm prevention. The really good news is that it has been reintroduced to the market at a significantly discounted price. There is also a $10.00 off coupon being offered by the manufacturer for a six month supply. Sentinel is an excellent heartworm prevention that offers good intestinal parasite coverage as well as a form of flea control. It does not kill adult fleas, but it does make them incapable of reproducing. Sentinel is a good choice for heartworm prevention when cost is an important factor.