Marijuana: Health effects of recreational and medical use

Wynne Armand, MD
Wynne Armand, MD, Contributing Editor

Marijuana is widely used, especially in adolescents and young adults. In the US, there are about 20 million users (about 7.5% of people aged 12 or older).

Marijuana is a mind-altering (psychoactive) drug. Similar to hemp, it comes from the cannabis plant. The plant’s primary drug effects come from the chemical delta-9-tetrahydrocannabinol, or “THC.” Marijuana is smoked, “vaped” (inhaling the vapor), or ingested in foods or teas. Over the last few decades, the concentration of THC in the cannabis plant has been increasing. In addition, the plant extracts are more potent.

Short-term effects of marijuana

Marijuana is often used for its “high,” and can make people feel more relaxed, alter the senses, and cause time distortion. It can also impair thinking, judgment, coordination, and memory. In some people, it may lead to paranoia or anxiety. Physical effects may include bloodshot eyes, dry mouth, slurred speech, and increased heart rate.

These sensations may last up to four hours after inhaling marijuana smoke, or up to 12 hours if consumed, though performance impairments may last for much longer.

Long-term effects of marijuana

Smoking marijuana can irritate the lungs and lead to breathing problems like cough, excess mucus, bronchitis, and worsening of conditions like asthma and cystic fibrosis. It is not clear if smoking marijuana is linked to lung cancer. Marijuana use during pregnancy can also harm a baby’s brain development.

We are learning more about the long-term effects of marijuana, and this leads to even more questions as we realize there is a lot we don’t know. What is clear is that some of the long-term effects correspond to heavy use. However, heavy use is difficult to define given the nature and limitations of past studies, the different ways marijuana and its extracts are used, and the shifts in THC concentration in cannabis.

The biggest health risks appear to be in those who start using as teenagers or young adults when their brains are still developing. In youths who use heavily, studies have shown deterioration in their learning and cognitive thinking that persists into adulthood, even after they have stopped using marijuana. There also appears to be a link between marijuana use and certain mental illnesses. Though most marijuana users will not develop dependence and addiction, it is more common in teenage users.

Medical benefits of marijuana

Medical use of marijuana (“medical marijuana”) is somewhat controversial. In the United States, current FDA-approved medications containing synthetic THC are dronabinol (Marinol capsules, Syndros liquid) and nabilone (Cesamet). These drugs are prescribed as appetite stimulants in AIDS patients and for chemotherapy-related nausea and vomiting. They are sometimes used off-label for similar symptoms in other conditions. An oral spray nabiximols (Sativex) is approved in other countries for treatment of multiple sclerosis muscle spasticity and cancer-related pain not managed by other pain medication, and is undergoing further evaluation in the U.S.

Although marijuana can help relieve the symptoms of certain medical conditions, its use is still illegal on a federal level, and because we don’t have data from large, long-term, well-designed studies, the FDA continues to have concerns about potential risks versus benefits. Fortunately, studies continue on marijuana, THC, and cannabidiol (a component of marijuana which does not have the mind-altering effects of THC). Early research shows cannabidiol and its derivatives hold potential promise in additional conditions like drug-resistant epilepsy and some psychiatric disorders (e.g., anxiety, substance use disorders, schizophrenia, and psychosis).

Some states have passed laws, contrary to federal laws, to allow the use of marijuana for medical conditions. Some states have also made the move to decriminalize marijuana use by adults or have similar measures on upcoming ballots. It is unclear what health impact decriminalization of marijuana would have. The experiences of other states and countries that have made these legal changes were varied and unpredictable. The greatest threat would be an increase in adolescent use.

Be cautious if you use marijuana

In the meantime, if you do use marijuana products, do not do so before driving or operating other heavy or dangerous equipment, and stop if you are pregnant. Talk to your doctor about the safety of continued use if you have a heart or lung condition, or are concerned that you might have developed an addiction. Avoid products that are packaged as candy or other edibles if there are children in your household who are more susceptible to a toxic overdose. And talk to your kids about the potential hazards of marijuana, because the risks are particularly high in young people.

Related Information: Harvard Health Letter


  1. Citizen Jay Daily

    Anyone who condones a prohibitionist view of Cannabis is a racist. The only reason Cannabis was made illegal was racist. It continues to be illegal due to racism. That is the truth at the base of the matter. All these arguments about the “safety” of using Cannabis are stupid and moot. We KNOW this. So stop it.

  2. sarajay

    Thank you for this article. You can tell it is effective based on how hot the pro pot people get when you impede their efforts to subvert public health so they can get high.

  3. nicolas giangospro u of minn

    For a university I cannot believe you did not mention CANNABINOID HYPEREMESIS SYNDROME what is the problem with society and universities in learning and teaching? Nixon and the previous collegiate academicians must have been drunk instead of calm and peaceful on an herb!

  4. Dr. Worm

    I think one thing that’s really missed in descriptions like this is a sense of how the effects of weed compare to the effects of alcohol. Reading this article you might get the impression that alcohol and weed are virtually interchangeable, i.e., weed users pose a threat to society equivalent to alcohol users. Which is wildly untrue, of course–an experienced user consuming weed in a normal manner will never become more disoriented than someone who’s only had a beer or two, no matter how much they smoke.

    • American Freedom Revolution 2016

      Legalize Marijuana – STOP Republican terrorists and alcoholic Nazi thugs.

      STOP the arrest and incarceration of 750,000 Americans each year.

      STOP the GOP war against Cannabis consumers in 2016.

  5. Wynne Armand

    This blog was not intended as propaganda, but rather to acknowledge that there are some who have health benefits from marijuana (or components of cannabis), to underline the importance of ongoing research areas, and to take note of possible risks and situations when marijuana (if used) should be used with caution. One such area of concern is heavy adolescent use. Discussing the possible risks is important, especially with children. As noted, past experience with decriminalization of marijuana has been variable: some places noted an increase in use, whereas other places did not find an increase in adolescent use. (Actually, I personally would support decriminalization of marijuana, but that does not belong in the body of the text.)

    • De Oppresso Liber

      In 1937, the Nazi Forces in America criminalized marijuana as part of the “Final Solution” policy to justify the unconstitutional arrest and incarceration of African-Americans and Hispanics.

      In 2016, WE THE PEOPLE will vote to legalize Marijuana and STOP the Nazi-era fascism and oppression imposed by a few Republican alcoholics.

      God bless America, God bless Liberty, Vote for FREEDOM 2016..!

  6. End prohibition

    Law enforcement, politicians, and the medical community are continuing to lose trust and respect thanks to the pants wetting, Reefer Madness ideology behind a relatively benign plant which is far safer than alcohol, tobacco, OTC, and prescription pharmaceuticals. The great thing about cannabis, is regardless if you’ve been brainwashed by last century’s ridiculous, racist propoganda, using cannabis won’t cause death, disease, or serious illness. In fact the health benefits far out weigh any side effects. Dr. Lester Grinspoon, a Harvard Psychiatry professor featured in the documentary, Culture High (check it out on Netflix), could educate dr. Armand about the disasters of prohibition, and the devastation created through the government’s war on drugs. So you can either educate yourself to the truth about cannabis, or keep your good standing with big Pharma and political prohibitionists. The people know. Vote out prohibitionists. Yes on 4!

  7. Arjun Chatterjee

    Not much she can do as an MD, she has to toe the line whatever her private wishes may be.

    It has come to a point thanks to the internet where the general public will just laugh if you throw normal seventies,eighties,nineties propaganda at them. So they have to get creative and continue balancing the public perception as well as pharma demands. Therefore, they try to appear to write a balanced article where they are not explicitly condemning marijuana, but they generally nudge you in that direction.

  8. Elise

    This article has been brought to you by The Alcohol Industry.

    • STOP Harvard Alcoholics Association

      The Harvard Nazi thugs as usual pushing their brand of Propaganda.
      Legalize Marijuana and STOP these scumbags in Boston..!

  9. Pamela McColl

    Stop way before you are pregnant as the damage to the developing brain of your baby is early on starting at two weeks gestation. If you are planning on starting a family pass up marijuana products and that applies to both men and women given the preliminary science that shows dna damage that is generational – meaning this could impact not only your offspring but the health of your grand-children. We are facing a 100 year problem in terms of damage to dna thanks to the recent pot heyday.

    • McIntosh

      Ms McColl, by “preliminary science” do you mean a new propaganda twist that hasn’t yet been supported by a highly biased and seriously flawed study that’s funded by an obviously prohibitionist oriented group?

      And congratulations on your invention of this “100 year problem”. You must spend a lot of sleepless nights thinking-up such nonsense.

    • Thomas

      Please cite your source.

      There is a study out of Australia released back in May that makes the claims you state. However, the study is inconclusive with many information gaps, and the methodology is not reliable enough to make the assertion you, or the study authors, claim.

      I do agree more research should be done (as we should for all things), but that does not mean we need to get into a reefer madness mentality, just as your post likens. It is irresponsible of you to be flailing such statements as if they were true without supporting evidence.

      I did leave a comment prior pointing to an article highlighting the fore mentioned study, but that seems to have not been posted.

  10. H C

    This article is full of too many errors to count. At least get your facts straight about cannabis. The author’s bias is obvious throughout this article.
    I expected so much more from what I had thought was an educated, science-based publication.

  11. Kevin

    This article seems to have not been thoroughly researched. A few notes:
    1) THC is one of the primary active ingredients in medical marijuana; however, there also exist entire sets of strains grown to be high in other cannabinoids such as CBD. CBD counteracts & complements the effects of THC. It is also worth noting the role of terpenes in producing the various differing effects per strain. There is tons of research in this area and its not mentioned at all.
    2) The author neglects to mention the primary problem with long term studies of MMJ users is determining if reduced mental capacities over time is due to correlation and not causation. I.E. those heavy users come from different backgrounds than a casual user. Those with traumatic childhoods are more likely to use MMJ to relieve their anxiety.
    3) “Toxic overdose” from edibles? There is no toxic overdose level. You cannot die directly from MMJ the same way you can from say alcohol poisoning. There is only short term mental discomfort from incorrect dosing. This perhaps the most shocking thing to read from Harvard Medical School.

    • Will

      Forgive me if I listen to the Harvard medical doctor over some guy who really doesn’t know what he’s talking about. Children are not small adults. They can die from ingesting a number of things that don’t harm adults. Their physiology is different. I can’t stand “know-it-alls” who have no training and no education. and for the most part don’t have two extra brain cells to rub together to make any sort of legitimate counter argument.

      • mary

        Will, with intending no disrespect to the doctor, being a ‘medical doctor’ doesn’t automatically make you an expert on Cannabis, it is plainly obvious to anyone with knowledge of this plant that Kevin knows more on the topic than the doctor who wrote this article, “No training and no education”, what a grand assumption to make about someone you have never met and “no legitimate counter argument” yet you have picked such a small shred of a comment, based on your own lack of knowledge and pure confusion, a comment that only contained facts. If you got off your stuck up ass and did some real research you might find it would theoretically, take 1,500 tonnes of Cannabis consumed in 15 minutes to induce a lethal overdose. This is physically impossible in every way.

        When you refer to ‘Marijuana’ you seem to be talking about some crazy drug, but your own body produces it’s own compounds identical to ones of the plant from ‘Endo Cannabinoid systems’ located in several places. These Cannabinoids regulate immune system functions, appetite and pain management, which explains why increasing evidence suggests the plant treats a massive range of illnesses, including many, many instances where children suffering from hundreds of seizures a day have found it to be the only cure. People around the world are also finding that they can effectively treat their own cancer with high doses of THC and CBD combined everyday, along with a change in diet.

        Human beings have been using this medicine for thousands of years, with the earliest recorded use in 2727 BC, it is a primary medicine in many cultures today and was found in 99 percent of mainstream medicine in the 1800’s to 1900’s, along with the booming hemp industry. Only during the introduction of a pharmaceutical drug corporation did the US government begin to create propaganda, from a movie claiming it turns people in to psychopathic murderers to the word ‘Marijuana’ (When they spread the lie that it was making Mexican immigrants rape American women) Yet with 0 recorded deaths in the history of human kind, and people overdosing everyday from prescription pills, it remains illegal under federal law, despite 25 states medically legalised. (But it’s a multi million dollar industry) Would you believe all that? And you said Kevin didn’t know what he was talking about…

    • Wynne Armand

      Thank you for taking the time to share your thoughts.
      Allow me to clarify that toxic overdose with children refers to taking too much such that it causes ill-effects.

  12. Blaine

    And masturbating frequently will make hair grow on the palms of your hands, cause brain damage, and send you to HELL as a sinner! Come on! Is this the REAL HARVARD we think it may be? or just another scam web site! spouting biased misinformation!
    Obviously this writer has never experienced the health benefits folks derive from it’s use. Big Pharma does not want a FREE WEED to undermine it’s HUGE PROFITS!

  13. Thomas

    Hemp is a variety of cannabis; not the other way around as you suggested.

    Hemp is a distinct strain of cannabis with a significantly lower concentration of THC, and other characteristics, than the commonly referred medical and recreational varieties.

    It’s difficult to take your article seriously when you confuse something as basic as this aspect of cannabis.

    Lastly, as already stated, the term “marijuana” has a disgusting history of racism tied to it. Yes, it is commonly used today in reference to cannabis. That doesn’t mean we should continue to refer to cannabis as such.

    • Wynne Armand

      Thank you, Thomas, for clarifying the classification. You are correct in that the hemp plant is a strain of Cannabis, generally with lower THC content. I will forward edits to make this clearer.

      • mary

        Erm, what about the word ‘Marijuana’? It was created by the US government in order to propagate racism towards Mexican immigrants. You should all start saying Cannabis! This is 2016.

  14. McIntosh

    This article is nothing more than hearsay that sounds like it’s from the 1930s. You can clearly see the authors bias early on when she puts quotes around the word “high”. As far as cannabis affecting coordination or causing time distortion how does the author explain so many pro athletes who use it? She’s probably one of those that believes Michael Phelps could have made something of himself if he never tried cannabis.

  15. Dave K

    What a disgusting article from someone that should know better. She warns of a toxic overdose with kids knowing fully well that the pharmaceutical poisons that she prescribes kill kids while this plant does not kill by overdose. The entire article is not informative but rather intentionally misleading.

  16. Nik Outhre

    Author has no clue. There goes credibility of this blog.

  17. Slawek Wojtowicz, MD

    I am sad that a physician from a respectable academic institution did not take time to educate herself about Cannabis. Incidentally name “marijuana” was coined by one of the most disgusting corrupt, racist, bigoted US government officials in history: Harry J. Anslinger, the father of DEA, who initiated campaign of lies (fueled by money from cotton manufacturers) that led to criminalization of a healing plant, sacred to multiple religions – from Hinduism to Christian Gnostics. It has been used for millennia for healing and spiritual purposes. In effect, the United States is engaging in one of the most vicious religious discrimination campaigns in history.