I just did a bigger reply that wasn't there after I hit the back-button because the server was busy.

But, this one answers your question.

Marijuana produces no withdrawal symptoms no matter how heavy it is used. It is habit forming (psychologically addictive), but not physically addictive. The majority of people who quit marijuana don't even have to think twice about it. Comparing marijuana to addictive drugs is really quite silly.

For a drug to be physically addictive, it must be reinforcing, produce withdrawal symptoms, and produce tolerance. Marijuana is reinforcing, because it feels good, but it does not do the other two things. Caffeine, nicotine and alcohol are all physically addictive.

http://www.erowid.org/plants/cannabi...aq1.shtml#3-20 (top of page)

Oh, and mice and rats won't self-administer THC, also.