Q. I have a collie mix puppy that is approximately 4 months old and weighs about 20 lbs. She ate a confortis flea pill that was meant for a 60 to 120 lb
DAVID ELBEZE, DVM, MRCVS
A. there is a high chance of toxicity, especially because she is a collie. Many collies carry the MDR-1 gene which makes them more sensitive to some medication such as ivermectin and the active ingredient in the comfortis tablet.
In general Comfortis tend to cause relatively a lot of side effects, especially if given in a higher dose than it should. If you have a collie which carries the MDR=1 gene she is in even bigger danger.
You should take her to the vet to induce vomiting (if pill was taken less than 3 hours ago) and be treated with fluids and activated charcoal.