![medical books about the effects of marijuana on body and brain medical books about the effects of marijuana on body and brain](https://static.scientificamerican.com/sciam/cache/file/38311867-BF57-4E57-8C79B58A86F1739E.jpg)
Most studies agree that factors such as genetic predisposition, how often and how much drug is typically used, age at first use, concurrent use of other drugs or alcohol, and other factors may contribute to a marijuana dependency.
![medical books about the effects of marijuana on body and brain medical books about the effects of marijuana on body and brain](https://image.slidesharecdn.com/truth-about-marijuana-booklet-en-120819214612-phpapp01/95/truth-aboutmarijuanabookleten-10-728.jpg)
The fact that many states have legalized marijuana use for medical purposes, and in some cases for recreational use, does not mean use of the drug is without risk. Of those, about 4 million users fit the criteria for a marijuana use disorder, meaning they are unable to stop taking the drug even in the face of negative consequences. The 2015 National Survey on Drug Use and Health reported approximately 22.2 million people used marijuana in the month prior to the survey. Although use of the drug was criminalized in the 1930s, it quickly became the most popular illegal drug in the country.
![medical books about the effects of marijuana on body and brain medical books about the effects of marijuana on body and brain](https://www.frontiersin.org/files/Articles/491828/fphar-10-01481-HTML/image_m/fphar-10-01481-g001.jpg)
Marijuana first became popular in the United States as a recreational drug in the early 20th century.