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Some people believe that smoking marijuana (without any tobacco) is harmful to your health and may cause problems. Examples of things I've heard weed supposedly causes:

  • All the health problems of tobacco (cancer, strokes, etc)
  • You get addicted to it and can't stop
  • It fries your brain
  • If you smoke too much you can die

I know for a fact that the last one is false, as the scientific consensus is that an overdose of marijuana in a human being is for all intents and purposes impossible. I'm not sure what "frying your brain" even means, and I don't know about the first two.

Has it been scientifically proven that weed is, or isn't, harmful?

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    "I know for a fact" - you shouldn't put your own unreferenced claims in the question (in addition to the claim you are being skeptical about) without references. Also, does anyone have a reasonable link to show the original claim is notable?
    – Oddthinking ♦
    Commented Oct 19, 2011 at 0:29

6 Answers 6

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It might be that smoking or ingesting marijuana has positive/neutral effects on certain health issues and neutral/negative effects on others. The consensus of studies are generally inconclusive in this regard.

For example, there are studies that say there is a 100% full increase in the odds of suffering from schizophrenia after trying cannabis

Others that say:

  1. "We hypothesized that there would be a positive association between marijuana use and lung cancer, and that the association would be more positive with heavier use," he said. "What we found instead was no association at all, and even a suggestion of some protective effect."

  2. "Earlier work established that marijuana does contain cancer-causing chemicals as potentially harmful as those in tobacco, he said. However, marijuana also contains the chemical THC, which he said may kill aging cells and keep them from becoming cancerous." - Link to Washington Post Article

and more recently:

Moderate Marijuana Use Does Not Impair Lung Function, Study Finds

Other studies have been conducted on the specific interaction between THC and cancer cells. This study, conducted by the Journal of Clinical Investigation states that:

...THC can promote the autophagic death of human and mouse cancer cells and provides evidence that cannabinoid administration may be an effective therapeutic strategy for targeting human cancers.

Again, I am not overlooking the potential health risks that have been proven to be associated with marijuana use, but I think it is a complex issue and media reports typically focus on potential negative health effects of marijuana use without mentioning potential positive health effects.

I think this graphic helps illustrates my point about the media reports:

Deadliest Drugs

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    Very interesting chart. Quite shocking actually how much reports are misleading. But chances of death being close to 0 doesn't mean harm to health is close to 0 too. I still would't advocate smoking weed to protect yourself from cancer, even if cannabinoid administration may be an effective therapeutic strategy for targeting human cancers. :)
    – Egle
    Commented Mar 11, 2011 at 9:03
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    Your answer is amazing. Do you think you could comment on the studies who showed a negative link too? That would make your answer super amazing.
    – Borror0
    Commented Mar 11, 2011 at 15:21
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    @Borror0 absolutely, when I get a chance I will come back to this question. I have looked at countless studies and oftentimes their focus is marijuana abuse, not marijuana use. There is an important distinction to be made there, and while the studies themselves may be conducted properly, they should not be used to represent the effects of marijuana use in general. I think this is mainly a problem with the question that was asked. Rate/quantity of consumption does not correlate linearly with negative health effects. The question should be more specific on rate of consumption.
    – Eratosthenes
    Commented Mar 13, 2011 at 0:19
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    I would guess the methadone death-count is slightly exaggerated. Methadone is used almost exclusively by people trying to break heroin addictions. A user may be trying to break the addiction exactly because he's in poor health. Also, the death may have been caused not by the methadone but by a relapse to heroin, or by effects of withdrawal. Finally, since methadone is typically be prescription, the fact that a given dead person is taking methadone can be easily determined.
    – Michael Lorton
    Commented Mar 26, 2011 at 2:02
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    @rmx -- I was surprised by what you wrote, but my very quick skim of the literature seems to back you up: it isn't the fall that kills you or the sudden stop, but getting back up. Somebody tries to stop using, hates withdrawal, goes and gets as much smack as he can and dies from the overdose. Anyway, my only point was that methadone will be known (since it's prescribed, not bought on the black market) whereas alcohol, benzodiazapine, and heroin have to be tested for. Do they even do autopsies on junkies they find in alleys?
    – Michael Lorton
    Commented Apr 27, 2011 at 18:42
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  1. Weed can cause some health problems, but not greater than tobacco or alcohol. It all depends on quantity/quality and personal health. So for perfectly healthy persons occasional use may have no effect, but for persons with heart problems it can do real damage.
  2. Can result in dependence, but less than amphetamines, tobacco or alcohol.
  3. No.
  4. There are no deaths recorded caused only by using cannabis. But there are deaths associated with usage of weed (car accidents, choking on vomit, etc.)

Advisory Council on the Misuse of Drugs(UK) states that:

  • High use of cannabis is not associated with major health problems for individuals or society.

  • Smoking cannabis may be more dangerous than smoking cigarettes because it has a higher concentration of carcinogens.

  • Occasional use of cannabis is only rarely associated with significant problems in otherwise healthy individuals, with the main worry being impaired control of your movements. It can also disrupt the control of blood pressure and increase the risk of fainting. However, occasional use can pose significant dangers for those with heart and circulation disorders and for those with schizophrenia.

  • Regular heavy use of cannabis can result in dependence but its addictive potential is far less than amphetamines, tobacco or alcohol.

  • Cannabis impairs mental functions such as attention, memory and performance and so can be dangerous for drivers and those who operate heavy machinery but, unlike alcohol, it does not increase risk-taking behaviour.

  • The birth weight of children whose pregnant mothers smoked joints might be lower than expected due to carbon monoxide in the smoke. They also run a small risk of minor birth defects.

  • Cannabis is less harmful than other class B substances including amphetamines, barbiturates or codeine-like compounds.

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  • Smoking marajuana when underage can affect IQ and brain development in later age. This is the biggest risk to marajuana in my mind, since the others are so trivial. I would say it's worth siting: scientificamerican.com/article/…
    – dsollen
    Commented Oct 2, 2018 at 21:02
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There is some evidence that consumption of cannabis can cause shizophrenia in some people (see this BMJ editorial), the newest study about this connection claims

Cannabis use is a risk factor for the development of incident psychotic symptoms. Continued cannabis use might increase the risk for psychotic disorder by impacting on the persistence of symptoms.

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    Does it mean that it's only a risk factor for people with pre-existing schizophrenia symptoms?
    – smzs
    Commented Mar 7, 2011 at 22:05
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    @smzs No, as far as I interpret it it might cause temporary temporary psychotic experienced to become permanent. The risk seems to be much greater for people with a familiy history of shizophrenia.
    – Mad Scientist ♦
    Commented Mar 7, 2011 at 22:11
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    Correlation =/= causation. From your source - "Another possibility is that the association arises because people with psychosis use cannabis to self medicate their symptoms"
    – Syntax Error
    Commented Apr 6, 2011 at 5:06
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    @Syntax Error: The problem with that reverse hypothesis is that it presumes psychosis/schizophrenia is the cause of the cannabis use. Other studies have shown that this cannot be the case; psychosis does not precede cannabis use often enough to explain the correlation. However, it is still possible that the two share an (unspecified) cause.
    – MSalters
    Commented May 30, 2011 at 12:28
  • Depend on Set and Settings, Drug does not produce Schizophrenia itself, but it acts as a neuro-chemical key to free the nervous system from ordinary patterns, if person is not strong enough may develop schizophrenia and even lead to long-term ego/identity loss, but Urban Cannabis is low potency and not really effective hallucinogen.
    – nima
    Commented Feb 6, 2012 at 10:04
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Cannabis is generally pretty innocuous as far as psychoactive drugs go. It's not entirely without negative consequences, however. There seems to be reasonable evidence (mostly correlative, granted, but there are enough correlations to be worried) for memory deficits and for decreased reaction speed in the face of contradictory information.

(Linked-to studies picked almost at random from among a number that have similar findings.)

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I’m bothered with the ‘you smoke cigarettes and you smoke weed, if cigarettes cause cancer weed does’ anti-science here. There are studies suggest the opposite, that smoking weed can reduce some forms of cancer. I’m not aware of a single study demonstrating that pot smokers who don’t ALSO smoke tobacco have any increased cancer rate.

Marijuana use and cancer incidence (California, United States). Sidney S, Quesenberry CP Jr, Friedman GD, Tekawa IS. Source Division of Research, Kaiser Permanente Medical Care Program, Oakland, California 94611-5714, USA. Abstract The purpose of this retrospective cohort study was to examine the relationship of marijuana use to cancer incidence. The study population consisted of 64,855 examinees in the Kaiser Permanente multiphasic health checkup in San Francisco and Oakland (California, United States), between 1979-85, aged 15 to 49 years, who completed self-administered questionnaires about smoking habits, including marijuana use. Follow-up for cancer incidence was conducted through 1993 (mean length 8.6 years). Compared with nonusers/experimenters (lifetime use of less than seven times), ever- and current use of marijuana were not associated with increased risk of cancer of all sites (relative risk [RR] = 0.9, 95 percent confidence interval [CI] = 0.7-12 for ever-use in men; RR = 1.0, CI = 0.8-1.1 in women) in analyses adjusted for sociodemographic factors, cigarette smoking, and alcohol use. Marijuana use also was not associated with tobacco-related cancers or with cancer of the following sites: colorectal, lung, melanoma, prostate, breast, cervix. Among nonsmokers of tobacco cigarettes, ever having used marijuana was associated with increased risk of prostate cancer (RR = 3.1, CI = 1.0-9.5) and nearly significantly increased risk of cervical cancer (RR = 1.4, CI = 1.0-2.1). We conclude that, in this relatively young study cohort, marijuana use and cancer were not associated in overall analyses, but that associations in nonsmokers of tobacco cigarettes suggested that marijuana use might affect certain site-specific cancer risks.

Cannabinoids in the treatment of cancer Amy Alexandera, Paul F. Smitha and Rhonda J. Rosengren , a, Abstract Cannabinoids, the active components of the hemp plant Cannabis sativa, along with their endogenous counterparts and synthetic derivatives, have elicited anti-cancer effects in many different in vitro and in vivo models of cancer. While the various cannabinoids have been examined in a variety of cancer models, recent studies have focused on the role of cannabinoid receptor agonists (both CB1 and CB2) in the treatment of estrogen receptor-negative breast cancer. This review will summarize the anti-cancer properties of the cannabinoids, discuss their potential mechanisms of action, as well as explore controversies surrounding the results. Keywords: Cannabinoids; JWH-133; CB1; CB2; MAPK

[The endocannabinoid system as a target for the development of new drugs for cancer therapy]. [Article in Italian] Bifulco M, Di Marzo V. Source Dipartimento di Scienze Farmaceutiche, Università di Salerno, Istituto di Endocrinologia ed Oncologia Sperimentale del CNR, Dipartimento di Biologia e Patologia Cellulare e Molecolare, Università Federico II, Napoli. [email protected] Abstract Studies on the main bioactive components of Cannabis sativa, the cannabinoids, and particularly delta 9-tetrahydrocannabinol (THC), led to the discovery of a new endogenous signalling system that controls several physiological and pathological conditions: the endocannabinoid system. This comprises the cannabinlid receptors, their endogenous agonists--the endocannabinoids--and proteins for endocannabinoid biosynthesis and inactivation. Recently, evidence has accumulated indicating that stimulation of cannabinoid receptors by either THC or the endocannabinoids influence the intracellular events controlling the proliferation and apoptosis of numerous types of cancer cells, thereby leading to anti-tumour effects both in vitro and in vivo. This evidence is reviewed here and suggests that future anti-cancer therapy might be developed from our knowledge of how the endocannabinoid system controls the growth and metastasis of malignant cells. PMID: 12723496 [PubMed - indexed for MEDLINE]

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  • In contrast to tobacco: Association Between Marijuana Exposure and Pulmonary Function Over 20 Years jama.ama-assn.org/content/307/2/173.abstract "repeated measurements of pulmonary function and smoking over 20 years (March 26, 1985-August 19, 2006) in a cohort of 5115 men and women" "Conclusion Occasional and low cumulative marijuana use was not associated with adverse effects on pulmonary function."
    – user951
    Commented Feb 5, 2012 at 21:53
  • Cannabis is a well known herbal, I recommend Food of the gods by T.McKenna for brief history of psychoactive plants. There are evidence for treatment of Migraine maps.org/mmj/mjrusso.html, HIV medication mpp.org/reports/medical-marijuana-hivaids.html and more en.wikipedia.org/wiki/Medical_cannabis
    – nima
    Commented Feb 6, 2012 at 11:32
  • One thing that often gets overlooked when comparing to tobacco, as well, is the fact that one typically takes in a fraction of a joint or maybe a joint per day, vs 20, 30 to 40 cigarettes.
    – PoloHoleSet
    Commented Aug 8, 2018 at 14:26
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The National Institute on Drug Abuse (NIDA) admits that cannabis is not physically addictive, it is not a gateway to drugs, it does not cause cancer or emphysema, there is no evidence it negatively affects driving, there is no evidence it causes an early onset of psychosis, and there is no evidence it negatively affects teen IQ.

The most important of these is the fact that there is no evidence cannabis negatively affects driving, because the main argument against cannabis legalization is “impaired driving”. It is the only reason anyone could claim to be a victim of cannabis consumption, yet it’s unfounded and doesn’t make anyone a victim anyway.

The second most important is that it doesn’t negatively affect teen IQ. This study was done after the popular “marijuana makes kids lose 8 IQ points” nonsense.

Quotes from NIDA:

In NIDA’s article Is marijuana a gateway drug? it says:

“These findings are consistent with the idea of marijuana as a ‘gateway drug.’ However the majority of people who use marijuana do not go on to use other, ‘harder’ substances.”

In NIDA’s article Drugged driving, they mention this study about cannabis and driving:

https://www.nhtsa.gov/staticfiles/nti/pdf/812117-Drug_and_Alcohol_Crash_Risk.pdf

The study says:

“The more carefully controlled studies, that actually measured marijuana (THC) use by drivers rather than relying on self-report, and that had more actual control of covariates that could bias the results, generally show reduced risk estimates or no risk associated with marijuana use (Elvik, 2013).”

And it concludes:

“The results of this study are in line with the previous research on the effects of marijuana on the risk of crash involvement. While a number of previous studies have shown some increased risk associated with marijuana use by drivers, many studies have not found increased risk. As was noted previously, studies that measure the presence of THC in the drivers' blood or oral fluid, rather than relying on self-report tend to have much lower (or no elevated) crash risk estimates. Likewise, better controlled studies have found lower (or no) elevated crash risk estimates.”

In NIDA for Teens’ article Marijuana, they mention a major study done on teenage twins. NIDA says:

“However, two recent twin studies suggest that this decline is related to other risk factors (e.g., genetics, family, and environment), not by marijuana use itself.”

The study can be read about here:

http://www.sciencemag.org/news/2016/01/twins-study-finds-no-evidence-marijuana-lowers-iq-teens

It says there is no evidence that cannabis consumption with teens causes a lower IQ or any developmental problems.

In NIDA’s article What are marijuana’s effects on lung health? they say:

However, while a few small, uncontrolled studies have suggested that heavy, regular marijuana smoking could increase risk for respiratory cancers, well-designed population studies have failed to find an increased risk of lung cancer associated with marijuana use.

And:

Animal and human studies have not found that marijuana increases risk for emphysema.

The rest of the article is full of maybes without any solid evidence or examples.

The DEA factsheet for cannabis still says:

“No death from overdose of marijuana has been reported”

According to the information provided by NIDA, the cannabis vegetable is safer and healthier than the sugar that we sell to children.

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