So, within the last month 16 dogs have been abandoned close to my house. My mother and I are trying to find homes for them. When we took a few of them to a local "charity" clinic for vaccinations and heart worm testing we were told that we can use Ivomec as a monthly heart worm preventative. The dosage is 1/10cc per 10lbs. You can buy Ivomec at your local co-op for about $25-$30 per 50ml. That is WAY cheaper than buying Interceptor or Heartguard. Ask your vet about it.