Ivermectin is good cheap way to prevent heart and some digestive system worms. I start my dogs on ivermectin at 6months old and give them a dose every 3 months. Don’t give it to a older dog unless you have had it tested and results were negative for heart worms. The dosage is 1cc per 100lbs. All of my dogs except the pups are roughly 50lbs so I just give them all 1/2 a cc straight down their mouth every 3 months and the 2 months in between I worm them with Panacur or Valbazen for digestive system worms.