Although antibiotics are slightly ot, your comment highlights the exact reasoning used to illegalize certain drugs: namely that drug use "has a definite negative effect on the society at large." Now I realize that a lot of people will argue this point, but that argument is mainly one of degrees (i.e. x or y drug isn't as bad as p or q drug). I don't think anybody in their right might would assert that drug use in general has no negative effect on the society at large.
Now given that drug X does have a negative effect on the society at large, there are really only two valid arguments for keeping them legal. Argument 1: personal freedom outweighs the negative effect on the society at large. Argument 2: the cure is worse than the disease.
Personally I believe all of the above. Some drugs should continue to be illegal. Other drugs should be allowed because the negative effect on the society at large is fairly low and is trumped by personal freedom. Yet other drugs would be better off illegal, but because of their prolific use the level of crime resulting from the trade trumps the negative effect on society at large.
Now by all means, continue to argue about which drugs fit into which categories.