I had to explain to one workplace that I was taking a medication as Rx'd that was on their list if no' for situations you just described. I passed the drug screening because it was prescribed. So, after training I did not operate heavy machinery for the reason you just brought up. My team lead didn't mind. One of my friends explained to me that the only thing that matters was my test compared to my hiring tests. I preferred to do other tasks anyway. I didn't trust that place, and I certainly didn't trust the people at the top.