According to the Journal of American Medical Association, 50 percent of the people who have a severe mental condition also have a substance abuse problem. JAMA also found that 37 percent of alcoholics have a mental illness, as do 53 percent of drug addicts.
When people use marijuana regularly, the drug may reduce thinking, memory and learning functions. Marijuana's effects on these abilities may last a long time or even be permanent. There have also been increases in fatal car crashes due to marijuana use in states like Washington and Colorado that have legalized the practice.