Most of the time they don't arrest you just because they suspect you may be under the influence of a drug that most likely does not affect driving negatively. Unless there is some evidence out there in the open, you won't go to jail for having taken any drug or being under the influence of a prescription med. The issue is that not rarely people take their meds and/or drugs with themselves -- in case you are searched (which does not happen that often) -- that is when you might get really in trouble.