Regardless, driving while impaired is a terrible idea. Although we have good tests to determine if people are under the influence of alcohol, no such tests are currently available for marijuana, making enforcement more difficult.
Babies born to women who smoke pot during pregnancy are more likely to be underweight, delivered premature and admitted to a neonatal intensive care unit, according to a 2016 systematic review. But there were no links seen for changes in birth length, head circumference or congenital malformations. There’s limited evidence for pregnancy complications for mothers, and there’s not enough evidence to comment on much else about babies and their outcomes.
Memory and concentration
There’s moderate evidence, from many studies, that learning, memory and attention can be impaired in the 24 hours after marijuana use. There’s limited evidence, however, that this translates into worse outcomes in academic achievement, employment, income or social functioning, or that these effects linger after the pot has “worn off.”
The possible relationship between marijuana use and mental health is complicated. The most recent meta-analysis found that there’s a significant connection between heavy marijuana use and a diagnosis of psychosis, specifically schizophrenia. This mirrored the findings of previous reviews that sought to cover only high-quality studies. Another systematic review highlighted a potentially small but statistically significant link between marijuana use and the development of bipolar disorder. Heavy users of pot are also more likely to say they have suicidal thoughts.
What makes this complicated is that it’s hard to establish the arrow of causality. Are people who smoke pot more likely to develop mental health problems? Or are people with mental health problems more likely to smoke pot?
There’s a similar issue when talking about the relationship between using pot and other substances. Some see marijuana as a “gateway” drug, leading to other substance use or abuse. Others see this as only a correlation in which people who are likely to use or abuse substances are more likely to use pot as well.
As states legalize the drug for general use, more cannabis users feel freed from secrecy. They smoke more in public, raising worries about secondhand smoke. A two-year-old study made news recently by arguing that one minute of exposure to pot smoke impaired lung function for at least 90 minutes, a greater impairment than from tobacco. This was a study in rats, though, not of humans out in the world. As for risk of a “contact high,” the amount of THC detectable in secondhand smoke is negligible.