I support the use of drugs for medicinal purposes.
I agree, that long-term effects of marijuana are lower than that of alcohol and in worst case scenarios, not worse than alcholism.
The fact is that both alcohol and marijuana when not used in moderation contribute to less productive individuals, broken homes and can become gateways to even worse substances. While under the influence inhibitions are lowered and criminal activity becomes more likely.
Barring our halachic requirements for alcohol, I see more of a reason to make alcohol illegal then to make marijuana legal.