Come to think of it, smoke has never been the threat but the users are the greatest threat.
How to you regulate its use?
I don't believe alcohol sits in the same chair with cannabis if you ask me but you will agree with me that people drink and find themselves in ditches or even go ahead to commit several offences .
Marijuana is there as a plant and can be used positively but when there is abuse of powers(this power is the power you have as a human being to make things work or to take that stuff or not)then there will be trouble.
Like you said, medical marijuana. This means that it has medicinal values and can be used for the purpose of treating or preventing or managing situations and should not be available across the counter.
Let us look at tramadol for example..it is now abused by many.
NSAID(Non0Steroidal Anti-inflammatory drugs) should not be available across the counter but you find that this still gets into the hands of those who should not have access to this.
In a nutshell I am saying it doesn't matter if legalized or not except the government as a whole manages other stuffs like NSAIDs, alcohol usage and all.