I'm not at all sure you can generalise about the effects of any drug, and individual case instances do not equate to a standard measure of effects, good or bad.
For example - my son-in-law drinks regularly, though not to excess.
If he were to drink a couple of pints of beer, and then drive his car home, the chances are he would arrive safely, with no apparent effects.
I am tee-total, and if I were to drink two pints of the same beer, I would not even be able to get the key into the ignition, much less start the car and drive home, i would be lying by the side of the car unable to stand.
Different drugs affect different people in different ways - which is the main issue with illegal drugs, strengths and content are competely ungoverend, with attendent overdosing and poisoning.
The legaisation of a drug for medical science can only be a good thing - it enables proper scientific testing and analysis of effects, which can hopefully lead to successful implenetation in treatment of a variety of conditions.
That is a long way from legalising a recreational drug like canabis - which is why the government is considering one, but not the other.