Effects of Marijuana on Human Brain Structure and Function: Doctor’s Perspective
With the expansion of cannabis reforms, more people around the world are paying attention to the high therapeutic potential of marijuana, particularly its possible use in treating severe and chronic illnesses. Currently, the effectiveness of cannabinoids in treating some chronic conditions previously considered incurable has already been proven: the use of marijuana in epilepsy therapy is now widespread globally, while the use of plant extracts for PTSD treatment has only recently begun to gain support in Western countries and regions.
Of course, skeptics point out the relatively under-researched topic of long-term side effects associated with marijuana use, which is difficult to refute due to a simple lack of reliable experimental data. Some sources claim that marijuana may negatively affect the development and functioning of young people’s brains, while other researchers report that regular marijuana use may increase the risk of psychosis and heart disease. Some even argue that marijuana contributes to the progression of neurodegenerative diseases like Alzheimer’s and multiple sclerosis, rather than alleviating their symptoms, contrary to some private research data.
To clarify the situation regarding the safety of regular therapeutic marijuana and plant extract use, both in the short and long term, we decided to consult a medical specialist who works with marijuana. Dr. Rebecca Siegel, a psychiatrist from New York who has been prescribing therapeutic marijuana for several years, agreed to share her experience and observations regarding the use of marijuana in treating various conditions, as well as its possible effects on human health and mental state.
“First of all, it’s important to note that many studies on both the positive and negative effects of marijuana refer to some ‘chronic form’ of use, without bothering to clarify what exactly is meant by this term,” she notes. “For this reason, it’s quite difficult to understand what is meant by chronic use in each specific case. Does it refer to daily consumption, or just weekly use? What concentrations of which cannabinoids are we talking about? Nothing is clear. Accordingly, it’s virtually impossible to judge how much the arguments of various studies, both negative and positive, relate to the real situation. In any case, I’m not aware of anyone conducting long-term observations, which is basically impossible, since even in the US and Canada, legalization only happened recently.”
Effects of Marijuana on Motivation, Motor Coordination, and Impulse Control
When discussing the possible effects of cannabinoids on the structure and functioning of the human brain, these effects should be divided into those that are short-term and those that manifest only in the long term. The first effects are felt by the user and their body either shortly after marijuana use or up to several weeks afterward. Long-term effects, in turn, may have consequences that are noticeable only months or years later, or even across generations.
“Regarding the short-term effects of marijuana use, it literally starts acting on the frontal lobes of the brain immediately after consumption,” Siegel notes. “Cannabinoids interact with receptors in brain areas responsible for motivation, impulse control, and higher neural activity, which manifests as reduced accuracy in motor coordination. Because of this, you should never drive or operate machinery after using marijuana.”
“SPECT brain scans (a type of scan that shows real-time neural activity in different brain regions) demonstrate that one of the short-term effects of marijuana use, observed for several days after intoxication, is a decrease in blood flow to the aforementioned brain areas, which may explain their reduced activity.”
“Similarly, scans show that cannabinoids also suppress activity in areas responsible for forming and recalling memories, which explains THC’s characteristic effect on human memory. Considering all these factors, it’s reasonable to assume that very frequent use of marijuana with high THC concentrations could potentially impair the overall functioning of these brain regions. Such impairments may be the result of cumulative cannabinoid effects, although it’s also possible that these patterns are related to other external factors. Unfortunately, at this time, the scientific community does not have enough reliable data, especially considering possible outside influences. Still, as a doctor, I strongly recommend everyone refrain from daily marijuana use for their own safety, at least until we can gather the necessary information.”
The Link Between Marijuana Use and Increased Risk of Psychosis
In her book “The Brain and Marijuana: What You Need to Know About the Effects of Recreational and Therapeutic Marijuana on the Brain,” Dr. Siegel mentions a study published by King’s College London in 2019. According to this study, people who use marijuana daily are about three times more likely to experience psychosis than those who do not use marijuana or use it less frequently. The risk of developing psychosis increases fivefold if the person regularly uses marijuana with a high THC concentration. Researchers classify any product containing 10% or more THC as high-THC marijuana. Accordingly, the vast majority of modern recreational strains and hybrids (for example, the average THC concentration in products sold in Colorado ranges from 17% to 28%) fall into this risk group.
However, Dr. Siegel believes that this apparent pattern is actually a coincidence, not a cause-and-effect correlation. “Perhaps doctors are simply asking the wrong question about the link between marijuana and psychosis,” she suggests. “We should ask how marijuana might positively affect psychosis, not just about the connection between cannabinoid use and this condition. In other words, it’s possible that people more prone to such disorders use marijuana more often and in larger amounts as a form of self-medication. That is, marijuana may be an unknown effective means of controlling psychosis symptoms, not the cause of their development and manifestation.”
“Some large-scale studies with big sample sizes and long observation periods support this position, but even these findings are ambiguous,” she notes. “Perhaps we’ll only get a clear answer about the link between marijuana and psychosis, and its effects, after widespread legalization allows doctors to study the effects of cannabinoids on the health of a truly large and diverse group of people.”
The Effects of Marijuana Use on Stress and Anxiety
On this topic, Dr. Siegel confidently stated that all possible sources confirm marijuana’s high effectiveness as a calming agent. She emphasizes that cases of increased stress when using marijuana are usually associated either with severe overdose or with smoking in an unfavorable environment that promotes stress and anxiety.
“Even marijuana opponents have always noted the plant’s strong sedative effect. Thousands of private studies and millions of online reviews also support this position. Of course, I personally can’t vouch for these properties of marijuana from my own experience: marijuana always stimulates me rather than relaxes me, so I’m somewhat surprised that some people use it not only for relaxation but also to increase their concentration on certain tasks. Nevertheless, the mass of data and observations speaks for itself.”
Given other possible risk factors of marijuana use, the doctor recommends using marijuana as a calming agent only in extreme cases, preferably with a specialist’s approval.
“Ultimately, as with other psychoactive effects of marijuana, it all comes down to the THC content in the product,” she notes. “The ideal sedative effect is achieved by balancing the levels of THC and CBD in the product. If CBD predominates over THC, marijuana will have more of a hypnotic than a sedative effect. If THC predominates, on the contrary, marijuana can strongly stimulate the nervous system, which can lead to temporary psychosis due to overdose. In this temporary state, there is no real threat to a person’s health, but excessive stimulation of the nervous system produces very intense bodily sensations, which, combined with the effects of cannabinoids on thought processes, creates the perfect environment for stress and paranoia attacks. Although in this state, subjects may be aggressive or frightened, it passes fairly quickly without any side effects except drowsiness and possible nausea.”
“In turn, I want to note that moderate marijuana use or the use of therapeutic CBD-containing products eliminates such manifestations,” the doctor adds. “CBD itself has a strong sedative and analgesic effect without the intense psychoactive impact characteristic of THC. For this reason, for relaxation, products with CBD are a more practical and safer alternative to THC-containing marijuana.”
The Risk of Marijuana Dependence
On this issue, Dr. Siegel could not give a definitive answer. While she believes that with frequent use or use of products with high THC concentrations, some people may develop psychological dependence on marijuana, she is highly skeptical about the existence of physiological dependence on the plant or its active components.
“Sometimes patients of different ages complain that they feel lethargic after quitting marijuana, thinking they are experiencing withdrawal from cannabinoids. However, according to all the data, this is purely a psychological phenomenon, the signs of which disappear just a few days after quitting. In my experience, I have never encountered cases where quitting marijuana affected the functioning of the body’s organs, as happens with some other recreational substances like alcohol or opiates,” she reports. “In the end, the phenomenon of ‘dependence’ is more related to the habits and psychology of each individual, not to any property of marijuana itself.”
Overall, the doctor does not oppose the use of marijuana for recreational or therapeutic purposes, only noting that any psychoactive substance should be used responsibly—specifically, not too often and not in large doses. “Cannabinoids have a cumulative effect. That is, their properties gradually build up, even when a person stops feeling them. In this way, these substances form tolerance in the body, which on average reduces their effect on the body’s receptors,” says Dr. Siegel. “For this reason, both recreational and therapeutic marijuana users should stick to a usage schedule to avoid ruining their own experience and to minimize possible risks.”
Additionally, the doctor advised young people to refrain from using marijuana or to use it no more than once a week. “The human brain continues to grow and develop until age 25. Given the possibility that cannabinoids can affect the activity and structure of neurons, I would not recommend people younger than this age to use marijuana frequently, for their own good. Although current observations do not prove a link between regular marijuana use and the development of certain diseases in old age, I would personally try to avoid such health risks. For the same reason, I believe that if therapeutic marijuana use is necessary, it’s best to consult an experienced doctor first to avoid causing more harm through self-medication.”