Scientists have learned a great deal about how THC acts in the brain to produce its many effects. When someone smokes marijuana, THC rapidly passes from the lungs into the bloodstream, which carries the chemical to organs throughout the body, including the brain.
In the brain, THC connects to specific sites called cannabinoid receptors on nerve cells and influences the activity of those cells. Some brain areas have many cannabinoid receptors; others have few or none. Many cannabinoid receptors are found in the parts of the brain that influence pleasure, memory, thought, concentration, sensory and time perception, and coordinated movement(5).
The short-term effects of marijuana can include problems with memory and learning; distorted perception; difficulty in thinking and problem solving; loss of coordination; and increased heart rate. Research findings for long-term marijuana use indicate some changes in the brain similar to those seen after long-term use of other major drugs of abuse. For example, cannabinoid (THC or synthetic forms of THC) withdrawal in chronically exposed animals leads to an increase in the activation of the stress-response system(6) and changes in the activity of nerve cells containing dopamine(7). Dopamine neurons are involved in the regulation of motivation and reward, and are directly or indirectly affected by all drugs of abuse.
Effects on the Heart
One study has indicated that a userÂ’s risk of heart attack more than quadruples in the first hour after smoking marijuana(8). The researchers suggest that such an effect might occur from marijuanaÂ’s effects on blood pressure and heart rate and reduced oxygen-carrying capacity of blood.
Effects on the Lungs
A study of 450 individuals found that people who smoke marijuana frequently but do not smoke tobacco have more health problems and miss more days of work than nonsmokers(9). Many of the extra sick days among the marijuana smokers in the study were for respiratory illnesses.
Other Health Effects
Some of marijuanaÂ’s adverse health effects may occur because THC impairs the immune systemÂ’s ability to fight off infectious diseases and cancer. In laboratory experiments that exposed animal and human cells to THC or other marijuana ingredients, the normal disease-preventing reactions of many of the key types of immune cells were inhibited(16). In other studies, mice exposed to THC or related substances were more likely than unexposed mice to develop bacterial infections and tumors(17, 18).
