Any drug missused can cause harm. What you have to do is look at the benefit vs risk profile for those who "should" be taking it.
Recreational marijuana is a completely different topic and conversation.
Medical marijuana has its place in clinical practice based upon the information that is becoming available. What I am interested in is the clinical practice of prescribing specific strains to address specific medical/clinical needs of the patient. That is where regulation and controlled prescribing is required. If this is not put into place, people will be self prescribing and buying product off the street, which may or may not benefit them for their particular need.
I've never known anybody ruin their lives with Marijuana, even total potheads I've known had real goals, like moving to Netherlands and owning a house on 1/4 an acre to grow Marijuana crops