The tide is turning on what the general public thinks of weed, as evidenced by more marijuana legalizations in the U.S., and will only continue to change. Part of this must be accredited to studies that have helped debunk marijuana myths such as weed being a gateway drug, weed being as addictive as heroin, or weed having negative effects on a user's physical and psychological health. One of the biggest myths has been that weed makes you dumb—as potheads way after Cheech and Chong have been characterized. A New UK study has disproved that. 

A study published in the Journal of Psychopharmacology on Jan. 6 is based on the findings among participants in the Avon Longitudinal Study of Parents and Children. For the ALSPAC 2,235 teenagers born in Bristol in 1991 and 1992 had their IQ tested when they were 8-years-old and then at 15-years-old. Almost 25 percent of the teenagers in the study had tried weed one or more times while 3.3 percent reported using it 50 times.

The study revealed teenagers who used marijuana had lower grades in school in addition to a lower IQ score. But University College London researchers who conducted the study said there wasn't proof weed directly caused those outcomes, adding that marijuana use just made it likelier for users to also smoke cigarettes, drink alcohol, and use other drugs.

"The notion that cannabis use itself is causally related to lower IQ and poorer educational performance was not supported in this large teenage sample," wrote Claire Mokrysz and her colleagues of the University College London. The researchers explained how other factors, such as alcohol use, could affect school performance and IQ.  "These findings therefore suggest that cannabis use at the modest levels used by this sample of teenagers is not by itself causally related to cognitive impairment. Instead, our findings imply that previously reported associations between adolescent cannabis use and poorer intellectual and educational outcomes may be confounded to a significant degree by related factors."

In a news released in 2014 before the study was published Mokrysz wrote, "People often believe that using cannabis can be very damaging to intellectual ability in the long-term, but it is extremely difficult to separate the direct effects of cannabis from other potential explanations. Adolescent cannabis use often goes hand in hand with other drug use, such as alcohol and cigarette smoking, as well as other risky lifestyle choices. It's hard to know what causes what—do kids do badly at school because they are smoking weed, or do they smoke weed because they're doing badly?"