I use 'Frontline for fleas and ticks and Ivomec for heartworm. Ivomec is much cheaper (we have at least 5 working Pyrs at all times)and my vet recommended it. Frontline has done well by us for 3 years - It usually works for at least 2 months per dose. Dan and Paula Lane Bountiful Farm Boer, Kiko and Spanish Goats Great Pyrenees LGDs Shady Point, OK