It has been a whirlwind century for the cannabis plant. In the first half of the 1900s, cannabis was largely demonized by American society as the “marijuana menace.” Buoyed by melodramatic anti-cannabis films like 1936’s Reefer Madness and by studies linking marijuana use to violent criminality, the federal government passed legislation in 1937 that made cannabis use illegal.     Now, well into our second decade of the 21st century, public opinion about cannabis is rapidly changing. About half of Americans have tried cannabis and a quarter of high school seniors have smoked marijuana in the last month. A recent article in Forbes predicts that “2016 will be marijuana’s big year,” and who could disagree? Twenty-three states have already legalized cannabis for medical purposes, many have decriminalized its use, and four states (Colorado, Washington, Alaska, and Oregon) have recently legalized recreational cannabis.

Cannabis has both benefits and risks.

The cannabis plant contains about 70 different cannabinoids, chemical compounds that affect our brain and help to create marijuana’s high. The best known of these compounds is called tetrahydrocannabinol, or THC. A dose of THC produces mild euphoria, stimulates appetite, fends of nausea, and relaxes, making it a useful treatment for people undergoing chemotherapy or dealing with chronic pain or terminal illness. (These effects also explain why cannabis is the third most popular recreational drug in the world, after alcohol and nicotine.) And THC is not the only potentially therapeutic chemical in cannabis: ongoing studies are looking at whether the compound cannabidiol can be used to treat seizures.   There is a common perception that cannabis is a safe, benign drug. But we need to keep in mind that it is a potent psychoactive substance that also comes with side effects and dangers. Users of both medical and recreational cannabis products should be aware of these risks and weigh them along with the drug’s potential benefits. The risks of cannabis use come in two categories: acute and long-term.

Cannabis turns up the noise in your brain.

A person under the influence of cannabis can experience perceptual changes that are eerily similar to those of psychosis: hallucinations, delusions, paranoia, feelings of unreality, deficits in problem solving, attention, and memory. In fact, researchers have noticed the parallels between the symptoms of marijuana intoxication and the symptoms of schizophrenia for a long time.

A recent study helps us understand what is going on in the brain of someone under the influence of cannabis. Using EEG, where electrodes placed on the scalp allow researchers to read the patterns of electrical impulses in the brain, researchers discovered that THC turns up the “noise” in the brain. To explain what this means, let’s use an analogy.   Imagine one computer transferring a copy of a video to another computer. The digital packets of information needed to render the video are transmitted along a cable as a series of zeros and ones. Once one computer receives this information from the other, it can decode it into the images and sound of the video. If the cable works well, the copied video is crisp and clean. If the cable doesn’t work very well, it may throw in a few extra zeros and ones here and there as it transmits its information. The receiving computer has no way to “know” which zeros and ones are real information and which are noise, so when it decodes the information it receives, the video it produces is pixelated and noisy.   This analogy may seem a far cry from what goes on in our brains, but in truth neurons actually communicate in a very similar way, using pulses of electricity. If you increase the number of random pulses going on in your brain cells, you can also disrupt your thoughts and ideas. THC apparently does exactly this, turning up the noise in your brain. This helps us to understand why a person under the influence of cannabis seems to have trouble disentangling reality from fiction and is slow at interpreting sights, sounds, and speech.

Cannabis can permanently impact your brain.

Until recently, most of our understanding of cannabis’s long-term effects on the brain came from anecdotes and from poorly controlled epidemiological data. These reports gave us worrisome indications of cannabis’s long-term effects: even after months (or sometimes years) of abstinence, cannabis users often have lower IQs, are slower to process information, suffer from memory issues and have trouble focusing their attention. More recently, better studies that took into account an individual’s cognitive ability before they started using cannabis have confirmed these long-term effects.   Not only does cannabis use probably affect memory and cognitive ability in the long-term, but it also increases the chances of long-term psychosis. It is estimated that cannabis use raises one’s relative risk of developing schizophrenia by about 40%.

If you’re under 25, beware.

As more research is being done on both the good and the bad of cannabis, we are finding that the drug’s effects are not merely temporary. Its changes to perception and memory, which are so obvious while intoxicated, seem to also linger in the brain for months or even years after use. The drug’s psychosis-like effects can also linger: it is estimated that cannabis is the cause of up to 14% of schizophrenia cases.   Worryingly, evidence indicates that the earlier you start using cannabis, the more pronounced its effects are on your brain. People who start using cannabis in adolescence are most likely to suffer from a stunted IQ and issues with impulsivity, problem solving, and planning.   So, if you are considering using cannabis medically or recreationally, discuss these risks with your doctor and consider other options. If you are currently using cannabis, consider stopping. The risks may be too great.