Marijuana: Effects of Weed on Brain and Body

Smoking marijuana produces euphoria and a range of psychological and physical effects that can be unpredictable at times.

Credit: Igor Kolos | Shutterstock

Marijuana is a combination of shredded leaves, stems and flower buds of the Cannabis sativa plant. Marijuana can be smoked, eaten, vaporized, brewed and even taken topically, but most people smoke it.

The intoxicating chemical in marijuana is tetrahydracannabinol, or THC. According to research from the Potency Monitoring Project, the average THC content of marijuana has soared from less than 1 percent in 1972, to 3 to 4 percent in the 1990s, to nearly 13 percent today. The increased potency makes it difficult to determine the short- and long-term effects of marijuana

How cannabis is consumed

In a 2010 National Survey on Drug Use and Health (NSDUH), 17.4 million people in the United States said they had used marijuana in the past month. According to the survey, marijuana is the most commonly used illegal drug. About 4 in 10 Americans have used marijuana at least once in their lives, according to the National Institutes of Health.

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Marijuana is usually smoked, according to the National Institute on Drug Abuse (NIDA). The cannabis (called "pot," "weed," "grass," etc.) is typically spread on rolling papers and formed into a cigarette, often referred to as a joint, or a cigar-like blunt. Smoking releases the THC, which is absorbed into the blood stream through the lungs. Glass pipes, bubblers and bongs are other ways to smoke marijuana.

Marijuana can also be ingested in food, often a choice of those who are using medical marijuana. Aside from the popular “pot brownie,” edible marijuana can be added to a number of foods, including candy, ice cream and butter. Some states that have legalized marijuana have issued rules for packaging and labeling "marijuana edibles."

Cannabis can be taken in liquid form, by brewing it as a tea. It can also be added to other beverages, including soda, milk and alcohol. Hashish is a resin made of the concentrated plant material. Other forms include capsules, oral sprays and topical oils. 

A relatively new method of inhaling marijuana is vaporization, a "smokeless" delivery system using devices such as e-cigarettes. By heating the cannabis at lower temperatures, the plant's oils or extracts are released. Several studies suggest that "vaping" is better for health than smoking pot. Vaporized marijuana contains little other than cannabinoids, according to a 2004 study in the Journal of Cannabis Therapeutics. Users inhaled fewer toxic compounds and carbon monoxide when vaping compared with smoking marijuana, according to a 2007 study in the journal Clinical Pharmacology & Therapeutics.

How marijuana affects the mind

Marijuana reaches the same pleasure centers in the brain that are targeted by heroin, cocaine and alcohol.

Depending on the quantity, quality and method of consumption, marijuana can produce a feeling of euphoria — or high — by stimulating brain cells to release the chemical dopamine. When smoked or otherwise inhaled, the feeling of euphoria is almost immediate. When ingested in food, it takes much longer, even hours, for the drug to signal the brain to release the dopamine, according to the National Institutes of Health.

Other changes in mood can occur, with relaxation frequently being reported. Some users experience heightened sensory perception, with colors appearing more vivid and noises being louder. For some, marijuana can cause an altered perception of time and increased appetite, known as the “munchies.”

The impact can vary by person, how often they have used the drug, the strength of the drug and how often it has been since they have gotten high, among other factors.

Other effects, according to the NIH, include:

• Feelings of panic, anxiety and fear (paranoia)
• Hallucinations
• Increased heart rate
• Trouble concentrating
• Decreased ability to perform tasks that require coordination
• Decreased interest in completing tasks

When coming down from the high, users may feel depressed or extremely tired. While marijuana use produces a mellow experience (users are sometimes referred to as “stoners”) for some, it can heighten agitation, anxiety, insomnia and irritability, according to the NIH.

Marijuana and teens

When marijuana use begins in the teen years, it can have a significant impact on brain development, including decreased brain activity, fewer neural fibers in certain areas and a smaller than average hippocampus, which controls learning and memory functions.

According to a Northwestern Medicine study of teen marijuana users, memory-related structures in the brain appeared to shrink, a possible signs of a decrease in neurons.

These abnormalities remained two years after the teen stopped using marijuana, indicating that the drug has long-term effects and look similar to brains of schizophrenics.

Those who started using marijuana after 21 generally do not experience the same type of brain abnormalities as those who started using the drug earlier.

Long-term users report that they sometimes have trouble thinking clearly, organizing their thoughts, multitasking and remembering things. Sustained marijuana use can also slow reaction times in some individuals.

How marijuana affects the body

Marijuana smoke can cause many of the same respiratory problems experienced by tobacco smokers, such as increased daily cough and phlegm production, more frequent acute chest illnesses such as bronchitis, and a greater instance of lung infections, according to NIDA.

While it had been thought that there was a connection between marijuana smoking and increased risk of lung cancer, even those who are heavy marijuana users do not appear to be at greater risk for lung cancer, according to a 2013 study by Dr. Donald Tashkin, UCLA professor of pulmonary and critical care medicine.

Marijuana can also raise heart rate by 20 percent to 100 percent shortly after smoking and the effect can last up to three hours, according to NIDA.

While it is widely thought that marijuana is not addictive, about 9 percent of users become addicted to marijuana. Long-term marijuana users who try to quit experience cravings, irritability, sleeplessness, decreased appetite and anxiety — some of the same physical symptoms of those trying to quit other types of drugs or alcohol. 

Several studies indicate that heavy marijuana use can lower the ability to fight infection and have an adverse impact on the immune system. Marijuana also can reduce sperm production in men and disrupts a woman’s menstrual cycle, according to NIDA.

Recent marijuana studies 

A 2016 study found a link between certain genetic markers and symptoms of marijuana addiction, suggesting that some people may have a genetic predisposition to marijuana addiction. That same study showed some overlap between the genetic risk factors for marijuana dependence and the genetic risk factors for depression, suggesting a possible reason why these two conditions often occur together, the researchers said.