drug.

The Harvard Mental Health Letter says:

Observational studies suggest that one in nine people who smokes marijuana regularly becomes dependent on it. Research both in animals and in people provides evidence that marijuana is an addictive substance, especially when used for prolonged periods.

This article from Psychology Today has a quote:

The Science of Marijuana is written by Leslie L. Iverson, a professor of pharmacology at the University of Cambridge in England. In the book, he reviews decades of international research on marijuana, both laboratory research and survey research. Based on his review of the scientific literature, between 10 to 30% of regular users will develop dependency. Only about 9% will have a serious addiction.

Here is another from Brown University:

No one would argue that marijuana is as addictive as alcohol ornevertheless had at times seemed to demonstrate an unseemly desire to control other peoples' lives, as well as angry reactions in personal disputes.

It is clear, for example, that Muhammad converted to Islam several years ago, but just changed his last name from "Williams" to "Muhammad" last year. It also appears that he spent 15 years in the US military, including stops at Fort Lewis Wash., and Fort Ord, Calif., and service in the Gulf War.

Several people, including law enforcement officials, said that both Muhammad and his stepson were known to speak sympathetically about the 9/11 hijackers.

Ideology may have played a part in Muhammad's actions, but as yet there is no proof. Among crucial unanswered questions: Why did he leave the military after fifteen years, with twenty years' service