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The Long-Term Effects of Marijuana on the Teen Brain

April 1, 2015

The Longterm Effects of Marijuana on the Teen Brain

In the 2012 Monitoring the Future study, about 45.3 percent of high school seniors reported having used marijuana once or more in their lifetime and about 23 percent used in the past month. 

It’s no surprise that marijuana is the second most abused drug by teenagers in the United States, after alcohol. With this high rate of use, many teenagers and some adults believe that marijuana is harmless, especially in states where marijuana is now legal. 

However, several studies are discovering common links between chronic marijuana use in adolescence and long-term physical, mental and behavioral problems that develop in adulthood. 

Cognitive Problems with Chronic Use


Memory Loss

A recent Northwestern University study found that 18 percent of young adults who chronically abused marijuana as teens performed worse on their memory tests than young adults who had never abused the drug. 

The same study also found that teens who smoked marijuana daily for about three years experienced a structural change in the brain that reshaped the hippocampus. The hippocampus is the part of the brain that controls memory function. Since teens’ brains are not fully developed yet, chronic use of marijuana may have a lasting effect on memory. 

Decrease in IQ 

There is also some evidence of declining IQ over time for marijuana users who began using in their teens and continued into adulthood. A study in the Proceedings of the National Academy of Science of the United States of America suggests a decline in brain function over time. 

In this study, they compared marijuana users and non-users in adolescence through age 38. They found that the adults who began using as teens and for many years after, decreased eight IQ points from childhood to adulthood; however many had low IQs to begin with. 

There are many conflicting studies out there regarding teen marijuana use and extensive research is needed. However, these studies suggest abnormal brain structure is linked to chronic marijuana use beginning in adolescence into adulthood. 

Short- and Long-Term Effects in Teens

Short-term Effects  

The short-term effects of marijuana use are well known and very visible. Depending on the potency of marijuana and frequency of use, every individual’s reaction may differ. Short-term effects include:

  • Dizziness
  • Dilated pupils
  • Red eyes
  • Acting silly for no reason 
  • Lack of motivation/interest
  • Slow response 
  • Being hungry and eating more

Long-term Effects

Multiple studies have found that teens who chronically use marijuana experience a structural change in the brain. This structural change contributes to more cognitive problems with prolonged use of marijuana such as:

  • Memory loss
  • Lack of attention and problem-solving skills
  • Lower intelligence 
  • Declining school/work performance (absences, poor grades)
  • Behavioral problems (increased promiscuity, DUIs, criminal behavior)
  • Mental health problems (moodiness, anger, anxiety)

Developing Research on Teen Marijuana Use

Since teens’ brains are still developing, the high levels of THC in marijuana has been shown to disrupt their normal brain functioning and may have lasting effects. While there is a lot of conflicting information about the effects of marijuana use in teens, more research is needed.  

The previous studies provoke many questions from other researchers. Longer research studies are in progress to determine whether chronic marijuana use in teens is linked to more permanent cognitive impairment in adulthood. 

Meanwhile, parents should be aware of these new developments as the long-term effects of marijuana may cause more alarming concern in teen development.

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