Saying we don't have a drug prohibition in effect, because some drugs are legal or even OTC, is like saying the Nazis weren't racist, because there were some races (for example, Aryans) who were not sent to the gas chambers as a matter of national policy.
Obviously, some drugs are prohibited, and if any are, then there is a prohibition in effect, specifically pertaining to those drugs.
Duh.
Which ones - THC perhaps?
Nope. It's availble via prescription (Marinol) ...
Maybe it's more a problem that *your* drug of choice is regulated (which you automaticlly then translate into 'a prohibition against') ...