It has been known for some time that collies and a few other herding breeds are more sensitive to certain drugs. This is because of a mutation in a gene known as MDR1. Animals with this mutation allow more of certain drugs to enter the brain with the potential to cause neurological side-effects. Early concerns revolved around heartworm medications.
The good news is doses at which common heartworm preventatives are given are safe for dogs even with the MDR1 mutation. Ivermectin, (the active ingredient in Heartgard), must be given at more than 16 times the preventative dose to cause problems in susceptible dogs. In fact, all of the heartworm preventatives are safe to 10- 20 times the labeled dose.
However, one common antidiarrheal drug to be wary of that is available over-the-counter is Imodium. I do not recommend the use of this drug in collies.
When in doubt, always check with your veterinarian before giving any drug. Just because it is safe for you does NOT mean it is safe for your pet. For more information call Hudson Veterinary Hospital at (330)650-2929.