This is a 12 pack of RSPCA Monthly Heartworm Tablets for medium dogs 23-44 lbs (10-20kg). This 12 month supply of heartworm tablets for medium dogs comes in 2 x green 6 packs. Each tablet will treat your dog for 1 month. These tablets prevent heartworm disease in dogs 23-44 lbs (10-20kg). It prevents heartworm disease in dogs by elimination the tissue stage of heartworm larvae that infected the dog in the 30 days before treatment. It can be used on puppies from 6 weeks of age.