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This is the ultimate question for many people. In fact, when discussing addiction, it is rare that the addiction potential for marijuana doesn't come up. Well, this is one of many posts on the topic, and before some of the readers get all upset, please read the post in its entirety.
Some basic points about marijuana
The active ingredient in marijuana, THC, binds to cannabinoid receptors in the brain (CB1 and CB2). Since it is a partial agonist, it activates these receptors, though not to their full capacity. The fact that cannabinoid receptors modulate mood, sleep, and appetite to some extent is the reason behind many of marijuana's effects - relaxation, increased appetite and euphoria.
But how is marijuana addictive? What's the link to heroin?
What most people don't know is that there is quite a bit of interaction between the cannabinoid receptor system (especially CB1 receptors) and the opioid receptor system in the brain. In fact, research has shown that without the activation of the µ opioid receptor (through a blocker like naloxone or naltrexone), THC is no longer rewarding. If the fact that marijuana activates the same receptor system as opiates (like heroin, morphine, oxycontin, etc.) surprises you, you should read on. The opioid system in turn activates the dopamine reward pathway I've discussed in numerous other posts (look here for a start). This is the mechanisms that is assumed to underlie the rewarding, and many of the addictive, properties of essentially all drugs of abuse.
But we're not done!
Without the activation of the CB1 receptors, it seems that opiates, alcohol, nicotine, and perhaps stimulants (like methamphetamine) lose some of their rewarding properties. This would mean that drug reward depends much more heavily on the cannabinoid receptor system than had been previously thought. Since this is the main target for THC, it stands to reason that the same would go for marijuana.
So what?! Why is marijuana addictive?
Since there's a close connection between the targets of THC and the addictive properties of many other drugs, it seems to me that arguing against an addictive potential for marijuana is silly. Of course, some will read this as my saying that marijuana is always addictive and very dangerous. They would be wrong. My point is that marijuana can not be considered as having no potential for addiction. As I've pointed out many times before, the proportion of drug users that become addicted, or dependent, on drugs is relatively small (10%-15%). This is true for almost all drugs. It seems to also be true for marijuana.
Citation:
Ghozland, Matthes, Simonin, Filliol, L. Kieffer, and Maldonado (2002). Motivational Effects of Cannabinoids Are Mediated by μ-Opioid and κ-Opioid Receptors. Journal of Neuroscience, 22, 1146-1154.
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