The Quick Facts on Marijuana

Caution: Be prepared for an interesting and possibly frustrating read, and be prepared to use your own discretion. Why? Because medical science is not a totally objective and exact science. And the research on marijuana is one of the most controversial subjects ever. There are lots of studies concluding that marijuana and cannabis is harmful in many ways, and lots concluding that it is safe and should even be used as medicine.

Table of Contents

Overview on the Research
Substances contained in marijuana
Storage in the body
Causes mental health problems
Opposing research on mental health
Impairs coordination and judgment
Opposing research on intelligence
Contributes to Accidents
Cannabis Poisons Children
Smoked Marijuana causes lung injury
Causes Cancer
Opposing research on Cancer
Addiction, Withdrawal and Gateway Drug
Opposing research on Addiction and Gateway Drug

(For the references in the text, hover over the reference number to see the summary. Click the reference number to get to the References section. Many will include links to the original source. Then click your browser back button to get back to the text.)

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Overview on the Research

There is evidence that some studies rely heavily on non-scientific anecdotes such as surveys and interviews. This is particularly true in the “marijuana is good for you” camp. One place where this is documented is the testimony of Dr. Janet Lapey during the U.S. House of Representatives Subcommittee on Crime – Medical Marijuana Referenda Movement in America on October 1, 1997. She documented some of the non-scientific anecdotes, including by Dr. Lester Grinspoon, who was on the board of the National Organization for the Reform of Marijuana Laws (NORML). (1)

This brings up another issue with the research, that of conflict of interest. How objective can your research be if you serve on the board of a pro-marijuana organization? (An interesting fact about NORML was included in Dr. Lapey’s testimony: In 1979, Keith Stroup, NORML’s founder, told an Emory University audience that they would be using the issue of medicinal marijuana as a red herring to give marijuana a good name. (2) (A ‘red herring’ is something that misleads or distracts from a relevant or important issue.)

Dr. Lapey also noted that he tobacco industry promoted cigarettes as “medicine” until the Federal Trade Commission halted the practice in 1955. (1 again.)

What really needs to be done is a whole research study on the methodology used in the research done on marijuana and cannabis. But for now, you will have to use your discretion when reading the research.

Substances contained in marijuana.

There are 33 cancer-causing chemicals contained in marijuana. Marijuana smoke also deposits tar into the lungs, up to four times as much as tobacco smoke. (3)

Marijuana smoke contains 50% to 70% more cancer-causing substances than tobacco smoke. (4)(5)

One major research study reported that a single cannabis joint could cause as much damage to the lungs as up to five regular cigarettes smoked one after another. (6)

Storage in the body.

THC, the active chemical in cannabis, is stored in the fat cells and therefore takes longer to fully clear the body than any other common drug. (7)

THC is stored in the blood for several months in regular users. (8)

Causes mental health problems

A definitive 20-year study into the effects of long-term cannabis use by Prof. Wayne Hall, published in 2015, found that Cannabis doubles the risk of developing psychotic disorders, including schizophrenia. (9)

In research done by Volkow ND et al. (10a) and Meier MH et al. (10b), marijuana is also linked to a reduction in alertness and self-conscious awareness, and a decline in IQ, especially when started at a young age. Marijuana has caused psychosis, depression, panic attacks, hallucinations, paranoia, hostility, depersonalization, flashbacks, decreased cognitive performance (i.e. thinking slower or not as clearly), disconnected thought, delusions, and impaired memory. (11)(12)(13)(14)

It affects children and teenagers much worse than adults

The negative effect of marijuana use on the functional connectivity of the brain is particularly prominent if use starts in adolescence or young adulthood.

A team from New Zealand found that those who had used marijuana on as few as three occasions by the age of 15 doubled their risk of developing schizophrenia by the age of 26. A study of 50,000 members of the Swedish army found that those who used marijuana heavily at age 18 were six times more likely to develop schizophrenia over the next 15 years. The risk doubled after using the drug only 5 to 10 times. (16)

One study found an association between frequent use of marijuana from adolescence into adulthood and significant declines in IQ (your intelligence level). (10b again)

Opposing research on mental health:

Federally sponsored population studies conducted in Jamaica, Greece and Costa Rica found no significant differences in brain function between long-term smokers and non-users. (17)

Impairs coordination and judgment.

Some of the short-term effects of marijuana are impaired body movement, difficulty with thinking and problem-solving, impaired memory, altered sense of time and changes in mood. (18)

Loss of coordination and poor sense of balance, and slower reaction times, along with intoxication. (19)

Opposing research on intelligence and cognitive decline:

The Canadian Medical Association Journal published an article in April 2002 which found that Marijuana smoking, even long-term, does not harm intelligence. (20)

A 1999 study of 1,300 volunteers published in The American Journal of Epidemiology reported “no significant differences in cognitive decline between heavy users, light users, and nonusers of cannabis” over a 15-year period. (21)

Contributes to Accidents

Marijuana is a major contributor to accidents. A study by Soderstrom CA of 1023 trauma victims revealed that marijuana had been used by 34.7 percent. (Refs 22). A study of 182 fatal truck accidents revealed that 12.5 percent of the drivers had used marijuana. (Refs 23) A study at the Einstein Trauma Center (Philadelphia) found 37% of admissions positive for cannabis. (Refs 24) A roadside study, conducted in Memphis, Tennessee, of reckless drivers not believed to be impaired by alcohol, found that 45 percent tested positive for marijuana. (Refs 25) A study conducted at the R Adams Cowley Shock Trauma Center, University of Maryland Medical Center, Baltimore, found that 31 percent of the 106 motorcyclists treated for injuries tested positive for marijuana. (Refs 26)

THC (the principal psychoactive constituent of marijuana) was found in the blood of more than 30% of fatally injured drivers. (27)

Cannabis Poisons Children

Research has documented pediatric poisonings caused by accidental ingestion of edible marijuana products. (28)

Smoked Marijuana causes lung injury.

Research shows that smoking marijuana causes chronic bronchitis and marijuana smoke has been shown to injure the cell linings of the large airways, which could explain why smoking marijuana leads to symptoms such as chronic cough, phlegm production, wheeze and acute bronchitis. (29)(30)(31)

Frequent marijuana-only smokers have more healthcare visits for respiratory conditions compared to nonsmokers. (32)

Causes Cancer

Marijuana causes cancer. This includes cancer of the lungs, mouth, throat and tongue, including metastatic epidermoid carcinoma in the lungs, head and neck carcinoma and upper aerodigestive tract malignancy. (33)(34)(35)(36)(37)

Opposing research on Cancer.

Donald Tashkin, a UCLA pulmonologist, hypothesized in a study funded by the National Institute on Drug Abuse that the chemical THC may prevent the cancer-causing chemicals in marijuana smoke from negatively affecting the body: “We don’t know for sure but a very reasonable possibility is that THC may actually interfere with the development of chronic obstructive pulmonary disease.” His study relied mainly on questionnaires filled out by certain groups. (38)

Addiction, Withdrawal and Gateway Drug

Many people say that Cannabis is not addictive. That is not true. The 20-year study by Prof. Wayne Hall showed that one in ten adults who regularly smoke the drug become dependent on it and those who use it are more likely to go on to use harder drugs. He also said that if cannabis is not addictive then neither is heroin or alcohol. Concerning withdrawal, he said: “It is often harder to get people who are dependent on cannabis through withdrawal than for heroin – we just don’t know how to do it.” (39)

There are also several other studies that show that Marijuana is addictive (40) and leads to the use of other drugs, such as cocaine. (41)

There is a whole organization of people who are working to stop their addiction to marijana: Marijuana Anonymous. Their website says: “Who is a marijuana addict? We who are marijuana addicts know the answer to this question. Marijuana controls our lives! We lose interest in all else; our dreams go up in smoke. Ours is a progressive illness often leading us to addictions to other drugs, including alcohol. Our lives, our thinking, and our desires center around marijuana—scoring it, dealing it, and finding ways to stay high.”(42)

Withdrawal symptoms include ones typical of other drugs, including difficulty concentrating, irritability, urges to use, difficulty sleeping, depression, vivid dreams, anger, headaches, sweating, coughing and decrease in appetite. (43)

Opposing research on Addiction and Gateway Drug

A new study has attempted to assess the risks of various drugs, such as looking at how addictive and toxic they are in terms of acute and chronic use, using a new technique called the “Margin of Exposure” (MOE) method. According to the results, cannabis is around 114 times less deadly than alcohol and was the only drug out of those examined to pose a low risk of death. (44)

According to Maia Szalavitz of, a recent study on rhesus monkeys suggests that being forced to take marijuana may actually make taking heroin less attractive and rewarding—and monkeys are a far closer model to humans than rats are. (45)



After reading the above, you may think it insane to legalize marijuana or to use it as medicine. Or you may think it is no problem at all. It is up to you to interpret the facts and views presented here, and to take action as they see fit. Actions speak louder than words.

“The only thing necessary for the triumph of evil is for good men to do nothing.” – Edmund Burke

Also: Do not assume that just because you have some scientifically researched facts about marijuana etc., whether you are in the pro or anti-pot camp, that sharing it with people in the other camp will convince them to change. Besides the conflicting research, some people have a stronger allegiance to a cause than they have to the truth.

Boiling frogs - smoking weed-a-500


Note: Some links to web pages change over time. The links below were valid at time of publication.

(1) Marijuana Subcommittee Hearing – Dr Janet Lapey in the U.S. House of Representatives Subcommittee on Crime – Medical Marijuana Referenda Movement in America on October 1, 1997. (Link)

(2) Emory Wheel, February 1979. (Marijuana Subcommittee Hearing – Dr Janet Lapey in the U.S. House of Representatives Subcommittee on Crime – Medical Marijuana Referenda Movement in America on October 1, 1997. (Link)

(3) Wu T-C, Tashkin DP, Djahed B, Rose JE. Pulmonary hazards of smoking marijuana as compared with tobacco. N Engl J Med 1988;318:347–351.)

(4) “Marijuana as Medicine: Consider the Pros and Cons,” Mayo Clinic, Aug 2006)

(5) National Institute on Drug Abuse, Infofacts [2004])

(6) “Additional Marijuana Facts,” University of Southern California, May 2015. (PDF Link)

(7)National Health Service website “NHS Choices.” (Link)

(8) “Additional Marijuana Facts,” University of Southern California, May 2015. (PDF Link)

(9) Hall, W. (2015), “What has research over the past two decades revealed about the adverse health effects of recreational cannabis use?” Addiction, 110: 19–35. doi:10.1111/add.12703.)

(10a) Volkow ND, Baler RD, Compton WM, Weiss SRB. Adverse health effects of marijuana use. N Engl J Med. 2014; 370(23):2219-27. (Link)

(10b) Meier MH, Caspi A, Ambler A, Harrington H, Houts R, Keefe RS, McDonald K, Ward A, Poulton R, Moffitt TE Proc Natl Acad Sci U S A. 2012 Oct 2; 109(40):E2657-64. – “Persistent cannabis users show neuropsychological decline from childhood to midlife.” (Link)

(11) Schwartz RH. Pediatric Clinics of North America 34: 305-317, 1987.

(12) Cherek DR et al. Psychopharmacology 111: 163-168, 1993.

(13) Andreasson S et al. Lancet 2: 1483 1485, 1987.

(14) Schwartz, RH et al. Am J Dis Child 143: 1214-1219, 1989.

(15) Effect of long-term cannabis use on axonal fibre connectivity. Zalesky A, Solowij N, Yücel M, Lubman DI, Takagi M, Harding IH, Lorenzetti V, Wang R, Searle K, Pantelis C, Seal M Brain. 2012 Jul; 135(Pt 7):2245-55. (Link)

(16) Andréasson S1, Allebeck P, Engström A, Rydberg U. Cannabis and schizophrenia. A longitudinal study of Swedish conscripts. Lancet. 1987 Dec 26;2(8574):1483-6. (Link)

(17) E. Russo et al. 2002. Chronic cannabis use in the Compassionate Investigational New Drug Program: an examination of benefits and adverse effects of legal clinical cannabis. Journal of Cannabis Therapeutics 2: 3-57.

(18) National Institute on Drug Abuse, February 2017. (Link)

(19) “Additional Marijuana Facts,” University of Southern California, May 2015. (PDF Link)

(20) The Canadian Medical Association Journal (Link)

(21) Media Awareness Project – Cannabis Use and Cognitive Decline in Persons under 65 Years of Age (Link)

(22) Soderstrom CA et al. Archives of Surg 123: 733-737, 1988. Marijuana and Alcohol Use Among 1023 Trauma Patients.

(23) Department of Transportation. National Transportation Safety Board Report, Washington D.C., February 5, 1990.

(24) Lindenbaum, G et. al. Patterns of Alcohol and Drug Abuse in an Urban Trauma Center, Journal of Trauma. V29( 12), 1654-59, 1989.

(25) Brookoff D et al, Testing Reckless Drivers for Cocaine and Marijuana, New Eng J Med 331: 518-522, 1994.

(26) Soderstrom CA et. al., Psychoactive Substance Use Disorders Among Seriously Injured Trauma Center Patients, JAMA. 277 (22): 1769–74, 1997 Jun 11.

(27) “Additional Marijuana Facts,” University of Southern California, May 2015. (PDF Link)

(28) Wang GS, Roosevelt G, Heard K. Pediatric marijuana exposures in a medical marijuana state. JAMA Pediatr. 2013;167(7):630-3.

(29) Tashkin DP. Smoked marijuana as a cause of lung injury. PMID:16128224 DOI:10.4081/monaldi.2005.64. NCBI, Jun 2005 (Link)

(30) Tashkin DP. Effects of marijuana smoking on the lung. Ann Am Thorac Soc. 2013;10(3):239-47.

(31) Howden ML, Naughton MT. Pulmonary effects of marijuana inhalation. Expert Rev Respir Med. 2011;5(1):87-92.

(32) Polen MR, Sidney S, Tekawa IS, Sadler M, Friedman GD. Health care use by frequent marijuana smokers who do not smoke tobacco. West J Med. 1993;158(6):596-601.

(33) Donald PJ: The chapter “Advanced malignancy in the young marijuana smoker” in Drugs of Abuse, Immunity, and Immunodeficiency; New York, Plenum Press, 1991.

(34) Ferguson RP et al. JAMA 261: 41 42, 1989.

(35) Taylor FM, South Med J 81: 1213 1216, 1988.

(36) Donald PJ Otolaryn Head & Neck Surg 94: 517-521, 1986.

(37) Donald PJ. Adv Exp Med Bio 288: 33-46, 1991.

(38) Washington Post, May 26, 2006. (Link)

(39) Hall, W. (2015), What has research over the past two decades revealed about the adverse health effects of recreational cannabis use?. Addiction, 110: 19–35. doi:10.1111/add.12703.

(40) Gold MS. Marijuana, NY: Plenum Medical Book Co., p.22 7, 1989

(41) Kleber HD. J Clin Psych 49:2 (Suppl), p. 3-6, 1988

(42) Marijuana Anonymous home page (Link)

(43) “Additional Marijuana Facts,” University of Southern California, May 2015.)

(44) Lachenmeier, DW. & Rehm, J. Scientific Reports 5, Article number: 8126 (2015) doi:10.1038/srep08126 (Link)

(45) Maguire D.R., France C.P. 2016 Apr;27(2-3 Spec Issue):249-57. Effects of daily delta-9-tetrahydrocannabinol treatment on heroin self-administration in rhesus monkeys. (Link)



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