What Does Weed Do to Your Brain? Long-Term Effects of Marijuana

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Cannabis goes by many names: pot, weed, marijuana, bud, and more. It's usually inhaled or consumed as an edible (like candy, brownies, or tea). It's still federally illegal in the U.S. But most states have decriminalized or legalized it. So now people of all ages can get cannabis more easily than ever.

With this in mind, it's no surprise that cannabis is the most commonly used illegal drug in the U.S. Almost 1 in 5 people used it at least once in 2019. But researchers still have a lot to learn about the potential risks and benefits of cannabis.

Among some of the biggest questions up for debate: How does cannabis affect the brain over time? Does weed kill brain cells? Can using weed permanently damage your brain? And what are the risks of starting to use cannabis as a teenager, or using it long term?

Researchers have started to dig deeper into these questions. Here, we'll lay out what the research has uncovered so far about how cannabis affects the brain over time. We'll also talk about how it affects IQ, memory, mental health, and learning abilities.

What does THC do to your brain?

Tetrahydrocannabinol (THC) is one of the active chemicals in cannabis. It's responsible for most of cannabis's effects on the brain - also called "psychoactive" effects.

When you consume cannabis, THC gets into the brain. This changes overall brain activity, especially in areas that are involved in thoughts, feelings, and perception. Many people report the following short-term sensations from consuming THC:

• Stress relief

• A feeling of calm and relaxation

• Less anger

• Happiness

• Pain relief

• Altered perception

• Increased appetite

THC can also cause temporary unpleasant reactions, like:

• Problems with thinking and memory

• Difficulty making decisions or solving problems

• Problems with balance, coordination, and reaction time

• Anxiety, suspicion, and panic

• Hallucinations and delusions

• Disorientation

• A feeling of being disconnected from your identity

What's more, THC activates the reward system of the brain and causes it to release the chemical dopamine. Other activities that activate this system are often essential to survival - like eating and sex. The brain is more likely to want to repeat anything else that stimulates it - like cannabis use - in the future.

So, if you repeat this process over and over again, for many years, does it permanently change how your brain functions? Let's take a look.

What are the long-term effects of cannabis on the brain?

It's not clear. There's still a lot to learn about the long-term effects of cannabis.

To date, cannabis use has been linked to many brain-related problems. These include:

• Memory problems

• Thinking problems

• Lower IQ

• Addiction (cannabis use disorder)

• Depression

• Anxiety

• Psychosis

But so far, the evidence isn't conclusive. In many cases, it's not clear whether cannabis causes these problems, or if there are other factors involved. It may also make a difference how often you use cannabis - and at what age you start.

The takeaway? More research is needed before we can fully understand the long-term effects of cannabis.

Does cannabis permanently change your brain, or kill brain cells?

It's hard to say. Some studies have looked at whether the long-term effects of cannabis on the brain are reversible. But, so far, it's impossible to say whether cannabis kills brain cells. There just isn't enough clear evidence.

In one small study, the brain began to return to its normal functioning 2 days after participants stopped using cannabis.

But a separate review of studies found that cannabis may continue to cause problems in the brain 3 weeks after a person stops using it. Studies found that attention and memory pretty much returned to normal. However, decision-making, concept formation, and planning ability were still affected. People who smoked long term in heavy amounts had the most difficulty.

How does cannabis affect a developing teen brain?

Decades of research strongly suggests that cannabis isn't good for the developing teen brain. In general, the earlier a person starts using cannabis, and the more heavily they use it, the more likely they are to have problems later in life.

A major difference between teen and adult cannabis use is that the brain is still developing until age 25. During this period, the brain is especially vulnerable to outside influences, including chemicals like THC and alcohol.

Some studies suggest that long-term exposure to THC as a teenager or young adult may affect learning, memory, and attention. It may also disrupt how the brain builds connections between the regions of the brain that help control these functions.

At this point, it's not clear whether these effects are permanent or not. But a large study under way aims to settle the issue. The Adolescent Brain Cognitive Development Study (ABCD Study) is currently following over 11,000 U.S. children ages 9 and 10 into early adulthood. It'll measure how genetics and environment (including substance use) affect brain growth and development. Results are expected in the next few years.

Does starting to use cannabis as a teen lead to a lower IQ?

Possibly. There's some evidence that heavy cannabis use as a teen can lower IQ - and that these IQ points don't come back, even after stopping cannabis use. This claim comes from the Dunedin Study, a well-known 2012 study in New Zealand.

This study followed a group of over 1,000 people from birth to age 38 and tested the group's memory, attention, and IQ over time. People who used cannabis heavily in their teens and into adulthood had an average 8 point decline in their IQ. They also had problems with attention and memory.

But other studies haven't found the same connection. One study also tested participants' IQs between 9 and 12 years old, and again between the ages of 17 and 20. It found that IQ fell in both people who used cannabis and those who didn't.

Another study had similar findings.

These studies also raised questions about whether participants with lower IQs had other traits. For example, if they had traits like impulsivity and risk-taking. Or if they were raised in a family environment that influenced their decision to use cannabis.

Frequently asked questions

If you or someone you know struggles with substance use, help is available. Call SAMHSA's National Helpline at 1-800-662-HELP (4357) to learn about resources in your area.

Stix, G. (2023). Heavy cannabis use linked to schizophrenia, especially among young men. Scientific American

Stringer, H. (2023). How does marijuana affect the brain? Psychological researchers examine impact on different age groups over time. Monitor on Psychology.

Substance Abuse and Mental Health Services Administration. (2023). Know the risks of marijuana.

Szalavitz, M. (2012). Does marijuana use by teens really cause a drop in IQ? Time.

Volkow, N. D., et al. (2016). Adverse health effects of marijuana use. New England Journal of Medicine.

Weir, K. (2015). Marijuana and the developing brain. Monitor on Psychology.

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