Most narcotics and prescription medication carry warnings on them to cover situations like this. If the medication in question says to use caution or not to use heavy machinery when under the influence, then such care should be used. Tylenol with Codeine, for instance, specifically says:
Use caution when driving, operating machinery, or performing other hazardous activities. Codeine may cause drowsiness. If you experience drowsiness, avoid these activities.
So, such warning constitutes knowledge of the effects of the drugs (if you don’t even read the label or attached information for your medications (or parents of a minor), then you’re looking at other issues than just the sinfullness of driving under the influence). If the person knows that impaired behavior is a side effect of the medication, and is experiencing impaired behavior, unless there is extreme grave circumstances surrounding his needing to be driving he is committing a sin. He is willfully endangering his own life, as well as those around him.
If the person doesn’t respond to the sinfullness of the action argument, give him a real life situation. If, as he is leaving his home/apartment/a parking lot, a child runs out from between two parked cars, can he live with the fact that if he not had an impaired reaction time the child would have survived? And that knowing he was under the influence of medication would affect his legal situation as well? Basically, if the threat of eternal damnation doesn’t do it for him, see if killing a child and facing jail time is enough to make him not drive.
Drugs aren’t something to mess around with. Neither is driving. The combination of the two is quite often fatal, and is easily avoidable.