THC acts upon specific sites in the brain, called cannabinoid receptors, kicking off a series of cellular reactions that ultimately lead to the "high" that users experience when they smoke marijuana. Some brain areas have many cannabinoid receptors; others have few or none. The highest density of cannabinoid receptors are found in parts of the brain that influence pleasure, memory, thoughts, concentration, sensory and time perception, and coordinated movement.
THC in marijuana is readily absorbed by fatty tissues in various organs.
Generally, traces (metabolites) of THC can be detected by standard urine testing methods several days after a smoking session. In heavy, chronic users, however, traces can sometimes be detected for weeks after they have stopped using marijuana.
The Monitoring the Future Survey, which is conducted yearly, includes students from 8th, 10th, and 12th grades. In 2006, the survey found that 15.7 percent of 8th-graders have tried marijuana at least once, and among 10th-graders, 14.2 percent were "current" users (that is, have used within the past month). Among 12th-graders, 42.3 percent have tried marijuana at least once, and about 18 percent were current users.