TIME Marijuana

Six Ways Science Says Marijuana May Hurt Your Health

New Year Celebration
Partygoers smoke marijuana during a New Year's Eve party at a bar in Denver, celebrating the 2014 start of retail pot sales in Colorado. Brennan Linsley—AP

With the increasing push for the legalization of marijuana across the country, science is rolling out research on why pot may not be so harmless.

Boosters of marijuana legalization often speak about the relative harmlessness of the drug, especially when compared to alcohol and tobacco, which kill millions of people a year worldwide. But while the evidence suggests that pot is less damaging than some other legal drugs, the exact effects of marijuana on human health have not been well studied. Existing research is often limited in scope and rarely shows a clear causal connection.

But there have been some worrying findings, especially considering the increasing use of marijuana by American adults. A paper published this year in Forensic Science International, for instance, described two rare deaths of young men that were attributed to heart conditions resulting from marijuana use.

With legalization taking place in Colorado and Washington State, more research will now be possible. For now, here is a tour of what has been documented so far about marijuana’s negative effects.

1. It can interfere with learning

Marijuana interferes with the brain’s cannabinoid receptors, affecting cognitive functions such as movement, memory, and emotional control. A recent small study found that impairment in working memory occurs immediately after marijuana use. Subjects who received a higher dose of THC—marijuana’s main active chemical—took significantly longer to complete immediate recall and mental calculation tasks.

2. It can lead to dangerous driving

Pot impairs functions key to driving, including reaction time, hand-eye coordination and depth perception, a study by the University of Chicago reported. In the first full year after medical marijuana was legalized in Colorado, there was a 12% increase in traffic fatalities, according to data analysis by researchers at Columbia University. However, studies have not been able to provide consistent evidence to prove that the effects of marijuana cause an increased rate of collisions. According to a different study published in Accident Analysis and Prevention, the closest comparison to the culpability of accident when under the influence of marijuana is to a driver who has taken penicillin, anti-depressants or an antihistamine, which suggests marijuana’s effects have a nominal impact on accident risk. More research is needed.

3. It may harm the developing brain.

Although a causal connection has yet to be found, studies show regular marijuana use—once a week or more—can change the structure of the teenage brain. Marijuana affects memory and problem solving abilities, both of which can impact academic performance. Researchers from the Harvard School of Medicine and Northwestern University Feinberg School of Medicine surveyed a small group between the ages of 18 and 25 and noticed structural abnormalities in the brain, specifically in grey matter, the nucleus accumbens, and the amygdala, after recreational marijuana use.

4. It could make you panic.

Marijuana may alleviate anxiety for some, but for others it can cause it. THC can cause increased heart rate, poor coordination, or lightheadedness, which can be triggers for anxiety attacks. Some research suggests that people who frequently use of marijuana—and who started using it as young adults—were more likely to have anxiety disorders or depression. Whether cannabis use causes anxiety disorders, however, is not known.

5. It can be addictive.

One in 10 users exhibits symptoms of dependence, according to the American Psychological Association. Marijuana’s rate of dependence liability of 9% is comparable to that of anti-anxiety medications and is well under the liability rates of alcohol (15%) and tobacco (32%), according to a study by the Institute of Medicine. However, the reason why some become addicted and others don’t remains unclear. Genetic studies have suggested that the involvement, or lack thereof, of CB1 receptors in response to cannabis can influence the likelihood of addiction. The receptor gene has been found to have a connection to the development of dependence to drugs and alcohol. Studies done on animals suggest that cannabis triggers reward centers in the brain, including neurons that produce dopamine, which could also encourage continued use.

6. It can stress your heart

Marijuana-related deaths are so rare as to be treated as mythological by marijuana boosters, but a paper published this spring in Forensic Science International does describe the deaths of two healthy men, ages 23 and 28, who experienced heart trouble after using marijuana. “To our knowledge, these are the first cases of suspected fatal cannabis intoxications where full postmortem investigations, including autopsy, toxicological, histological, immunohistochemical and genetical examinations, were carried out,” the authors write. The authors surmise that the cardiovascular events were the result of increased heart rate that can happen in some pot smokers, particularly in the first hours after using marijuana. Nonetheless, the authors conclude, that the “absolute risk of cannabis-related cardiovascular effects can be considered to be low, as the baseline risk for most cannabis smokers is low and cannabis-induced changes are transient.”

Your browser, Internet Explorer 8 or below, is out of date. It has known security flaws and may not display all features of this and other websites.

Learn how to update your browser