Short-term use of the drug impairs thinking and coordination, and in long-term studies, teens who smoke marijuana have lower IQs later on, as well as structural differences in their brains, though scientists debate whether this is an effect of the drug or a result of habitual pot smokers seeking out less intellectually stimulating pursuits. Marijuana use has been linked to mental illnesses such as schizophrenia, according to NIDA. Marijuana smokers are also likelier to suffer from bronchitis, according to a 2014 New England Journal of Medicine review of marijuana’s health effects.