Marijuana plant

Talking about marijuana can be a little complicated. More and more states are legalizing it for medical and recreational use, so it’s less taboo than it once was—but the research is mixed when it comes to the benefits versus the risks. What we do know are some of the effects it has on your brain and body when you use it, and that’s key knowledge for making informed decisions.

How marijuana makes people feel varies—a lot

“When marijuana is smoked, tetrahydrocannabinol, or THC (the main active ingredient), and other chemicals quickly reach their target receptors in the brain, and the user experiences the effects almost immediately,” says Dr. Ruben Baler, neuroscientist and marijuana researcher at the National Institute on Drug Abuse, Washington DC. “Some of these include a sense of euphoria and relaxation, but pleasant experiences are by no means universal. Some users experience anxiety, fear, or panic.”

Recent research shows that students perceive marijuana as lower risk than those in the past did, which may have to do with increasing legalization.

“In terms of legalization, yes, there are chemicals in marijuana that have the potential to benefit patients with specific conditions—but legalized recreational use has given people the wrong perception that the drug is completely safe. More research is needed to pinpoint long-term effects, but what we do know is that THC can directly affect the way in which the brain processes information.”
—Dr. Ruben Baler, neuroscientist and researcher at the National Institute on Drug Abuse, Washington DC

Keep in mind that, like with alcohol, a person still must be age 21 or older to use marijuana in states that have legalized recreational use.

To cut the confusion, we collected the evidence. Click on the graphic below to find out the science on how the body reacts to marijuana.

Man infographic

Man infographic

Brain
Marijuana immediately triggers a boost of dopamine (a neurotransmitter that affects your brain’s pleasure center), which leads to those famous warm and fuzzy feelings (the “high”). But one 2016 study suggests that this may not be the case for everyone. The study—published in Molecular Psychiatry—found that heavy marijuana users (those who used at least twice per day, five days a week or more) had lower dopamine releases over time, which is associated with not only decreased happiness but also worsened memory and a struggle with performing tasks that require attention.

Overall, there isn’t enough evidence to make claims about the long-term effects of marijuana on the brain. However, a 2014 study in the Journal of Neuroscience found significant brain differences (in size and shape of areas associated with decision making) in the MRI results of 18- to 25-year-olds who smoked marijuana at least once a week (compared to people with little or no prior experience with marijuana).

Eyes
THC decreases blood pressure, causing capillaries and blood vessels to dilate. Besides the telltale red eyes (caused by increased blood flow) and the dryness that usually accompanies it, there’s some evidence to show that marijuana could actually damage your vision in the long run. According to a 2017 study in JAMA Ophthalmology, regular use has been shown to hinder how your retina cells processes visual information, which the researchers suggested could potentially alter vision.

Lungs
Unlike smoking tobacco cigarettes, researchers have not found a clear link between lung cancer and smoking marijuana. However, a 2008 study in Chemical Research in Toxicology found that marijuana smoke contains similar toxic chemicals to those found in tobacco cigarette smoke, such as ammonia (commonly found in pesticides) and hydrogen cyanide (a poisonous chemical compound). Interestingly—and confusingly—a 20-year study of more than 5,000 men and women suggests that regular marijuana smokers tend to have an increased lung capacity compared to non-marijuana smokers (JAMA, 2012). The main takeaway: We still don’t know exactly what’s going on with marijuana’s effect on the lungs, but research shows that regular marijuana smokers are more likely to experience wheezing, excess phlegm production, and a stubborn cough.

Heart
There’s lots of disagreement when it comes to marijuana and its effects on your heart. While one recent study that analyzed 20 million health records found a 10 percent increased risk of heart failure and a 26 percent increased risk of stroke among users, other long-term studies haven’t found a strong-enough correlation between marijuana use (with no tobacco) and any increased risk. One thing experts all agree on: More research is needed.

Appetite
One of marijuana’s most notorious side effects is also its most mysterious. Even some of the world’s top researchers can’t pinpoint exactly why marijuana often sparks a ravenous hunger (also known as the “munchies”) that leads people to raid the fridge or the nearest convenience store. So far, it’s believed that the key lies in the CB1 receptors within our brain, which trigger feelings of being full or hungry. THC seems to activate a hunger-promoting hormone that tells us we’re hungry, even if we aren’t.

Researchers have noted another strange phenomenon that’s not yet understood. Various studies show that marijuana users tend to have lower body weight than non-users, even when they have high daily calorie intakes, according to a 2014 review of research published in Innovations in Clinical Neuroscience.

The paper’s authors theorize that this may be because of variations in how the studies were conducted and how marijuana use was measured (e.g., short-term vs. long-term use), because food and marijuana may compete for the same reward system in the brain, or because there may be some still-unknown mechanism at work that actually prevents or slows down weight gain.

Edible marijuana products (such as baked goods, candies, and beverages) are popular in states that have legalized recreational and medicinal use (Methods Report, 2016). Edibles don’t carry the same risks associated with smoking (though such little research has been conducted on them, we really don’t know what effect they could have on our health). What we do know is that there are some stark differences in how THC affects the body when it’s consumed vs. when it’s smoked.

For example, when a person smokes marijuana, they usually start feeling the effects within 10 minutes, and the high lasts for about 4 hours. When a person consumes edibles, it takes at least 30 minutes to start feeling the effects; it peaks about 3 hours in, and lasts for about 12 hours (JAMA, 2015). This delay in onset and the much longer duration can make it difficult for people to figure out how much THC they’ve consumed or gauge how it will make them feel, which could be leading people to inadvertently consume too much. A 2015 study in JAMA found that most people going to the hospital in Colorado for marijuana intoxication (with symptoms like anxiety, panic attacks, and vomiting) had ingested edible THC products.

The takeaway: If you plan to consume edibles, start with a very small amount and wait a couple of hours to feel the effects before deciding to take any more.

Motor skills
Studies show THC majorly affects your reaction time and motor coordination, and it’s correlated with impaired driving. The number of car accident-related insurance claims in Colorado, Washington, and Oregon increased by about 3 percent since the three states legalized marijuana for recreational use, according to a report by the Insurance Institute for Highway Safety. In fact, aside from alcohol, it’s the drug most commonly detected in the blood of drivers who’ve been in a car crash, according to the National Institute on Drug Abuse (NIDA). Whether or not you choose to use marijuana, never get behind the wheel while you’re impaired (or get in the car with a driver who is). Not only could you be pulled over and charged for driving under the influence (which usually comes with a massive fine and license suspension), but you could cause a fatal accident.

Fertility

Whether or not you’re not trying to start a family anytime soon, keep in mind that a common cause of infertility is oddly shaped, confused, or “lazy” sperm. A 2014 study showed young men who smoked marijuana had higher levels of these wonky sperm and had fewer of the strong swimmers required to fertilize an egg and make a baby. This study pinpointed three months as the maximum affected period, and the long-term effects aren’t clear. The research on how marijuana use may affect women’s fertility is limited and still unclear. However, pregnant women who use marijuana may have twice as high a risk of having a stillborn baby compared to non-users, and there may be other detrimental effects on the developing fetus, according to NIDA.

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