Delta-9-tetrahydrocannabinol (THC) is the main active ingredient in marijuana, responsible for many of its known effects. When marijuana is smoked, its effects begin almost immediately. THC rapidly passes from the lungs into the bloodstream, which carries the chemical to organs throughout the body, including the brain. The effects of smoked marijuana can last from 1 to 3 hours. If marijuana is consumed in foods or beverages, the effects appear later—usually in 30 minutes to 1 hour—but can last up to 4 hours. Smoking marijuana delivers significantly more THC into the bloodstream than eating or drinking the drug.
Scientists have learned a great deal about how THC acts in the brain. THC binds to specific sites called cannabinoid receptors (CBRs) located on the surface of nerve cells. These receptors are found in high-density in areas of the brain that influence pleasure, memory, thinking, concentration, movement, coordination, and sensory and time perception. CBRs are part of a vast communication network known as the endocannabinoid system, which plays a critical role in normal brain development and function. In fact, THC effects are similar to those produced by naturally occurring chemicals found in the brain (and body) called endogenous cannabinoids. These chemicals help control many of the same mental and physical functions that may be disrupted by marijuana use.
When someone smokes marijuana, THC stimulates the CBRs artificially, disrupting function of the natural, or endogenous, cannabinoids. An overstimulation of these receptors in key brain areas produces the marijuana "high," as well as other effects on mental processes. Over time, this overstimulation can alter the function of CBRs, which, along with other changes in the brain, can lead to addiction and to withdrawal symptoms when drug use stops.
The THC content or potency of marijuana, as detected in confiscated samples over the past 30+ years (Potency Monitoring Project, University of Mississippi), has been steadily increasing. This increase raises concerns that the consequences of marijuana use could be worse than in the past, particularly among new users, or in young people, whose brains are still developing. We still do not know, however, whether cannabis users adjust for the increase in potency by using less or by smoking it differently. We also do not know all the consequences to the brain and body when exposed to higher concentrations of THC.