I believe marijuana just makes people stupid, robs them of ambition, etc.; that is their choice. Make it legal and tax it like cigarettes. .
And alcohol doesn't? Anything can rob a person of their intelligence or ambition if they abuse it, including alcohol and tobacco and marijuana. As long as it is restricted enough to not negatively impact the user's life, I don't see why marijuana should be singled out when alcohol and tobacco kill more people than anything else.