[Christ]

Everything can be physically addicting, especially psychoactive drugs. By artificially increasing or decreasing the amount of a certain neurotransmitter, you encourage the brain to stop producing those chemicals naturally. If you stop use of any drug after a prolonged period of use, you will be deficient in the effected neurotransmitter for a period of time, and during that period you will crave the drug.

It is also worth noting that THC encourages much the same chemical release/re-uptake inhibition as muscle relaxants, which, similarly, are very easy to get psychologically addicted to, but difficult to get physically addicted to.

The brain doesn't produce tetrahydracannabinol (THC) for itself. It produces small amounts of DMT but that's a whole other topic I won't get into. THC affects the cannibinoid receptors in the brain, causing changes in appetite, taste, emotion (typically more often pain, sometimes fear, happiness, etc.) and mood. Cannabidiol (CBD), a cannabinoid and the prime pain-killer in female marijuana plants itself is probably the most addictive psychologically but even in it's purest form, it is not physically addictive. The risk of physical addiction to THC is about as high as the risk of allergic reaction to the CBD even at 30%+ THC content.

Pic related, it's what I just smoked:

Spoiler:

http://www.medicalmarijuanablog.com/wp-content/uploads/2011/06/white-rhino-marijuana.jpg