THC or Delta-9-tetrahydrocannabinol

Young teenagers smoking marijuana which contains THC
THC is thought to be particularly harmful to teens. GARO/PHANIE Getty Images

THC, which stands for delta-9-tetrahydrocannabinol or Δ-9-tetrahydrocannabinol (Δ-9-THC), is a cannabinoid molecule in marijuana or cannabis that's long been recognized as the main psychoactive ingredient in marijuana—that is, the substance that causes users to experience the marijuana high.

But THC isn't the only psychoactive ingredient in marijuana. In fact, THC is just one of over 400 different active substances—and 60 different cannabinoid molecules—contained in marijuana, although THC is the most well-recognized.

Another important cannabinoid molecule that has received recognition is cannabidiol (CBD).

How Much THC Is in Marijuana?

THC was first isolated in 1964; the amount of THC contained in marijuana varies by the preparation or the way that cannabis is prepared for use, such as leaf/bud, hashish, or hashish oil.

The amount of THC in modern strains of marijuana has massively increased since it was isolated in the 1960s when marijuana was imported from Mexico and typically contained about 1-2 percent THC concentration. By the early 2000s, the concentration had increased to about 4 percent—between twice and four times as strong as it had been during the "hippie" movement, when recreational marijuana use, marijuana abuse, and marijuana addiction became widespread. And in 2012, the strength of modern "high potency" strains of marijuana, such as sinsemilla, or "skunk," is reportedly at least four times as strong, containing 16-22 percent THC.

Researchers have often been confounded by anecdotal and unreliable reports of the benefits of cannabis, which have frequently been ascribed to the effects of THC.

More recently, as the component molecules have been isolated and used experimentally, a better understanding has developed of the specific effects of each molecule.

How Does THC Work?

THC works by attaching to cannabinoid receptors, which have been mapped throughout the brain and nervous system. THC can be detected in the body much longer than most other drugs: although the psychoactive effects only last for a few hours, it can be detected in the blood up to 20 hours after ingestion, and it's stored in the body fat and organs for three to four weeks after ingestion. Hair follicle testing may identify THC after even longer periods of time. However, urine testing has been found to be an unreliable method of detecting THC.

Is THC Addictive?

Despite the belief by many chronic marijuana users that the drug is not addictive, THC tolerance and dependence have been demonstrated in numerous animal studies, and experimentally at the cellular level. It's interesting that heavy pot smokers or "stoners" will vehemently deny dependence, despite daily use, whereas cigarette smokers rarely have difficulty acknowledging their addiction—both to themselves and others.

The reason for this is unclear; it may relate to the relatively low activation of the neurotransmitter dopamine, which has been recognized as central to the reward cycle in many other drug and behavioral addictions. Then again, it's possible that the belief that marijuana is non-addictive rests on the myths about marijuana.

Is THC Harmful?

There is considerable research evidence that THC is associated with increased risk of developing psychosis, particularly among adolescents. A meta-analysis, which is a type of study that combines the results of many previous studies, found some evidence that THC may be neurotoxic, as there are differences in the brain structure of chronic marijuana users who do not have psychosis.

Research into the effects of THC is complicated by many factors, but it appears that there is sufficient evidence that THC can be harmful, particularly to younger people whose brains are still developing, and they should, therefore, avoid frequent use of marijuana.

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