"The other [explanation] is much more basic and probably the best one: It’s just a matter of dose. Most people who develop emphysema due to tobacco smoking do so after smoking one or two or three packs a day for decades. Think for a moment about what you’d be like if you smoked 40 or 60 joints a day for 20 or 30 years — lung damage would probably be the least of your problems. It could be that if someone were courageous enough to smoke that much marijuana...they would get lung damage. They probably wouldn’t care about that lung damage if they got it. But people generally don’t use that much."

4. MYTH: You can't get addicted to weed.

"On the flip side, a risk that many people don’t think about — that I certainly didn’t think about — that turns out to be very real is the risk of dependence and addiction. I honestly didn’t think that marijuana addiction was a real phenomenon, at least not in the same way that addiction to cocaine or heroin or even nicotine is, but it turns out it is.

"The same circuits that create addictions to any of those other drugs are also involved in creating addiction to marijuana, with the same syndrome of craving and continued use in a way that’s disruptive to work, relationships, and physical health. There’s also a withdrawal syndrome...for people who use a lot regularly and then suddenly stop. People develop anxiety, agitation, and sometimes aggression in much the same way as somebody who uses alcohol continuously and then stops using.

"It’s the same circuit that gets activated, and it works in the same way, but researchers also note that it’s not as likely to be activated. It’s very difficult to pin down the probability of addiction, because so many things determine it, but...somewhere around nine to 10% of regular users of marijuana will become dependent. That's compared to between 13 and 15% of [regular users] becoming dependent on other drugs, like cocaine.

"One lesson seems to be that your likelihood of becoming addicted to marijuana is less than that of [becoming addicted to] other drugs. And, when people do become addicted to marijuana, it’s not necessarily as severe an addiction. Many people are addicted to marijuana using clinical psychiatric criteria, but still manage to function.