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NIDA. (2012, October 9). Can Smoking Marijuana Actually Lower Your IQ?. Retrieved from

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By Carol Krause, Chief of NIDA’s Public Information and Liaison Branch
October 9 2012
Marijuana leaf overlaid with a down arrow and Q

Many teenagers assume smoking weed is harmless because of all the myths floating around saying it’s safe. What few people know is that the age you start using marijuana actually makes a difference. In fact, if you start smoking it as a teenager, there can be some serious problems for you down the road.

Although we already knew from past research that if you start smoking pot as a teen, you’ll be more likely to get addicted, new research (just published in a well-known journal called Proceedings of the National Academy of Sciences) now says if you smoke marijuana heavily as a teenager, it can actually lower your IQ!

Scientists looked at more than 1,000 people born in 1972 and 1973. When they were 13 years old, they were given IQ and other kinds of intelligence tests. They were interviewed every few years about their use of marijuana and then tested again when they were 38 years old.

The results? Those who smoked weed heavily as teens showed mental decline even after they quit using the drug—and had, on average, an 8-point drop in their IQ scores. An 8-point loss could push a person of average intelligence into the lower third of testers. Those who started smoking pot after age 18 also showed some decline, but not as much.

This was an interesting study because it also collected information from people who knew the study participants. They reported that people who smoked marijuana heavily had more memory and attention problems and did not organize their lives as well, misplacing things and forgetting to keep appointments, pay bills, or return calls. This highlights the lasting effect marijuana can have on the teenage brain, which is still developing and still wiring itself to handle the onslaught of information it gets every day. The toxic chemicals in marijuana can mess up that wiring process and hurt your ability to do well in school and in life.

Carol Krause is the Chief of the Public Information and Liaison Branch at NIDA. Since arriving at NIDA in 2006, Ms. Krause has launched several new innovative programs for teens, including Drug Facts Chat Day (an annual live Web chat between NIDA scientists and teens), National Drug Facts Week (to stimulate community events between scientists and teens), and the first Addiction Science Awards for high school participants in the Intel International Science and Engineering Fair.