Dear Andrea, Suki will be fine with Program Plus for heartworm prevention. Both products, Sentinel and Program Plus, are manufactured by the same company, Novartis, and have the same active ingredients: milbemycin oxime and lufenuron. The only point I need to make is that you need to ensure that she gets the correct dose based on her body-weight and that the treatments are carried on monthly at regular time intervals. Please note that this product also has shortages on the market at the moment and it seems that it will get worse so you might want to buy a year’s supply. Please read and retain the information leaflet before you treat your pet.