THC, or tetrahydrocannabinol, is the chemical responsible for most of marijuana's psychological effects. It acts much like the cannabinoid chemicals made naturally by the body, according to the National Institute on Drug Abuse (NIDA).

Cannabinoid receptors are concentrated in certain areas of the brain associated with thinking, memory, pleasure, coordination and time perception. THC attaches to these receptors and activates them and affects a person's memory, pleasure, movements, thinking, concentration, coordination, and sensory and time perception, according to NIDA.

THC is one of many compounds found in the resin secreted by glands of the marijuana plant. More of these glands are found around the reproductive organs of the plant than on any other area of the plant. Other compounds unique to marijuana, called cannabinoids, are present in this resin. One cannabinoid, CBD is nonpsychoactive, according to the National Center for Biotechnology Information, and actually blocks the high associated with THC.

Effects on the body
THC stimulates cells in the brain to release dopamine, creating euphoria, according to NIDA. It also interferes with how information is processed in the hippocampus, which is part of the brain responsible for forming new memories.

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THC can induce hallucinations, change thinking and cause delusions. On average, the effects last about two hours, and kick in 10 to 30 minutes after ingestion. Psychomotor impairment may continue after the perceived high has stopped, however.

"In some cases, reported side effects of THC include elation, anxiety, tachycardia, short-term memory recall issues, sedation, relaxation, pain-relief and many more," said A.J. Fabrizio, a marijuana chemistry expert at Terra Tech Corp, a California agricultural company focused on local farming and medical cannabis. However, he said, a study in the British Journal of Pharmacology found that other types of cannabinoids, as well as terpenes (compounds that produce flavor and fragrance in plants), can modulate and reduce negative effects.

Risks
The effects of marijuana make it a popular drug. In fact, it is considered one of the most commonly used illicit drugs in the world. But these effects also concern mental health advocates. THC can trigger a relapse in schizophrenic symptoms, according to NIDA.

Another possible risk of consuming THC comes in the form of impaired motor skills. Marijuana may impair driving or similar tasks for approximately three hours after consumption and it is the second-most common psychoactive substance found in drivers, after alcohol, reports the National Highway Traffic Safety Administration. People taking medical marijuana are instructed not to drive until it has been established that they can tolerate it and conduct motor tasks successfully.

The use of marijuana may cause problems for younger people, and long-term problems. "Some of the side effects of THC include a decrease in IQ, memory and cognition, especially in younger people," said Dr. Damon Raskin, medical director at Cliffside Malibu Treatment Center. "However, the jury is still out on long-term effects, as not enough research has been done on it yet. There is some speculation that it could impair fertility in men and women and also compromise a person's airways, but the studies are still not clear."

A study by the University of Montreal published in the journal Development and Psychopathology in 2016 found that early use of marijuana can affect teens. Smokers that start around age 14 do worse on some cognitive tests than non-smokers. The study on almost 300 students found that pot smokers also have a higher school dropout rate. Those that waited to start around age 17 did not seem to have the same impairments.