What drugs are you allowed to have in your system as a medical professional?

I'm a medical professional in the United States. I know that as a medical professional, you can't have illicit drugs in your system. However, if you're prescribed a drug by a medical professional, is that okay? If you had a drug in your system before you became a medical professional, but not after, is that okay? I'm just wondering since I heard that this is how medical professionals are drug tested to see if they're high on the job.