I did look at the internet first but I know already about mdr1.  What I can't find it how long before symptons are seen if the dog is sensitive to this product. Does anyone with a collie know this?

I would never use anything that has a derivative of Invermectin in it's components on any herding breed even if the dog had been DNA tested normal(or was normal by parentage)

The test is offered by several labs.

I wouldn't wait for the symptoms to show-these can include seizures & death