One study showed that young people who reported driving frequently while on marijuana were twice as likely to be involved in accidents. It seems that the marijuana-induced changes in perception and sense of time may be entertaining in the living room but can be deadly on the highway. Marijuana affects all of the skills that are necessary to drive safely. The effects include diminished capacity to accurately judge distances, slower reflexes, and distorted perceptions of how fast you are going. Combined with alcohol, the hazards of driving under the influence of marijuana are far more extreme than with either drug alone.
Addiction is a complicated process that is described by tolerance, dependence and withdrawal.
Users can develop a tolerance over time. People who smoke marijuana or who have a genetic predisposition might develop a tolerance and would need more of the drug. Withdrawal can occur after chronic use and may be mild due to the THC lasting longer in the body because it is fat-soluble. Some people report irritability and restlessness and some have insomnia and nausea. Heavy users however, can begin to show signs of dependence when they experience withdrawal symptoms.