Caffeine and marijuana are in the two bottom slots. People disagree which of these two drugs deserves to be fifth and which sixth. When an accomodation is made for the medicinal use of marijuana, it will be seen as a medicine which has less capacity for dependence than Valium. I have heard about possible infertility, loss of short term memory, etc. The only long-term effect of smoking marijuana which I am concerned about is the effect on the pulmonary system.
Cannabis is at least as dirty as tobacco smoke, from the point of view of the lungs.
That is, they take only as many puffs as they need to achieve the desired effect. It follows and it has been demonstrated in the lab that people inhale fewer puffs of a more potent marijuana than of a less potent marijuana. These devices, like other paraphanalia, are illegal now but the technology exists and they will be an important part of medicinal marijuana. I cannot be sure that such data may not be developed in the future, but at the present time there is no evidence to support that contention. I know a person who is having spasmodic crises several time a day and lasting sometime to 2 hour period.