I really never enjoy this slippery slope argument, when arguably the most harmful drugs [1, 2] are already very much legal (sometimes the drug is controlled, but at least studies can be done, and those who can benefit have access).
[1]:http://www.economist.com/blogs/dailychart/2010/11/drugs_caus... [2]: note Adderall ~= meth, morphine ~= heroine, tobacco, alcohol, you get the picture