In Europe, cannabis is the most illicit used drug, mainly for its psychoactive properties (becoming "high" or "stoned"). The active compound in cannabis is THC (tetrahydrocannabinol). In recent years the strength of cannabis (marihuana) available on the market in the Netherland has increased (especially "netherweed"). The THC-content has increased from on average 8.6% in 2000 to on average 20.3% in 2004. Experienced users adapt their habit of smoking to the strength of the cannabis sigaret (joint). However, there is also a group of people, mainly young ones, who intent to smoke the joint entirely, and can therefore be exposed to higher levels of THC. Information is lacking about the effects of cannabis at higher levels of exposure. Because of this, the Ministry of Health, Welfare, and Sports of the Netherlands ordered the investigators to perform a study to gain more insight in this issue.