If you own a particular breed it's important to know if they have a genetic predisposition to medication sensitivities. Collies, and a number of the other herding breeds, can have a genetic mutation called MDR1 which makes them sensitive to a lot of meds. Ivernectin, the main ingredient in heartworm meds can kill them.

I didn't know this until after Toby got neutered. He had a horrible time coming out of anesthesia. For 24 hours we had to drip water into his mouth. He couldn't get up to eat, drink or pee. Later we found out that he was mu/mu for MDR1 and that he's very sensitive to the anesthesia that they used. We had Cameron tested as soon as we got him. He's also mu/mu, so we had the vet use a different drug when he got neutered.