This is a 12 pack of RSPCA Monthly Heartworm Tablets for small dogs up to 22 lbs (up to 10 kg). This 12 month supply of heartworm tablets for small dogs comes as 2 x blue 6 packs. Each tablet will treat your dog for 1 month. These tablets prevent heartworm disease in dogs up to 22 lbs (up to 10 kg). 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.