Scientific White Paper on Marijuana Harm to Health


  • Marijuana is a mind-altering (psychoactive) drug from Cannabis plants that produces a euphoria or “high” in users.
  • Pro marijuana advocates are stating that marijuana is a safer alternative to alcohol consumption. This is not based on facts, and should be countered based on science
  • Public perception has recently shifted in favor of marijuana legalization 1.

Marijuana is typically consumed by smoking a dried portion of the plant or ingesting foods or extracts containing marijuana’s psychoactive compound tetrahydrocannabinol (THC). THC levels in marijuana have increased; marijuana purchased in 2013 is more potent than marijuana purchased a decade ago 2.

Negative consequences of marijuana use/abuse:
Marijuana use has been associated with negative psychological, physiological, and behavior effects. Short term effects of marijuana use can include diminished psychomotor skills (e.g., driving) and difficulties in thinking, learning, and problem-solving. Long term effects of marijuana use can include addiction and withdrawal 2. Recent studies and news articles have highlighted an additional emerging health concern with legalized marijuana: unregulated pesticide residues and other potentially-hazardous contaminants. Marijuana production and certification is largely unregulated and pesticides contaminate 10-35% of marijuana that is tested. These contaminants pose dangers – especially to immune-compromised patients 3,4,5.

  1. Marijuana and its negative impact on the brain:
    Marijuana advocates have incorrectly suggested that marijuana has properties that can protect the brain6. Several scientific studies have shown that THC is the main psychoactive component of marijuana 7 and can affect brain neurodevelopment8. THC targets regions of the brain associated with memory, concentration, pleasure, perception, coordination, movement, problem-solving, and other functions 7.
    Adolescents are particularly susceptible to the effects of cannabis 9. A recent study showed that persistent marijuana use before age 18 was associated with diminished intelligence at age 38 – the decline persisted even after the individual had ceased using marijuana10. Early marijuana use is also associated with increased risk of presenting some psychiatric illnesses; however a causative mechanism has not been demonstrated 8.
  2. Marijuana addiction and health care burden:
    THC can produce addiction symptoms (e.g., reinforcing effects and increasing tolerance)2. In 1992 more than 92,000 Americans were treated for marijuana use 11,12. In 2011, the number increased nearly tenfold to 872,000. Additionally in 2011, 4.2 million Americans were classified as marijuana dependent – more than pain reliever and cocaine dependence combined 13. Marijuana addiction and treatment translates into a substantial health care burden in America. In 2008, the National Institute of Drug Abuse estimated that addiction to illegal drugs including marijuana costs $181 billion annually 14.The estimated healthcare burden of marijuana treatment in the state of California in 2009 was $25.6 million 15.
  3. Marijuana and cancer:
    Marijuana smokers have an increased risk of cancer of the head, neck, lungs, and respiratory tract 7. In 2009 16, California EPA added marijuana smoke to the Proposition 65 list 17 as a “chemical known to the state to cause cancer” and listed 33 unique chemicals in marijuana smoke that are carcinogens 18; many of these carcinogens are also present in tobacco smoke 19.
  4. Marijuana and the heart:
    Marijuana use is associated with arterial disease such as stroke, myocardial infarction, and sudden cardiac death, especially in young people without any medical history 20,21,22.

Marijuana, public perception of safety and driving:
Marijuana use increased in the US from 2008 to 2010 and there was a concurrent decrease in the public’s perception of risk, including among 12-17 year olds 23. There is a common misconception that driving under the influence of marijuana is not unsafe 24,25. In 2012, California Office of Traffic Safety published results of a study showing that:

  • 5.5 % of drivers tested positive for marijuana
  • 14.3% of drivers surveyed admitted to having smoked marijuana within two hours of driving in the past year.
  • 40% of drivers have used marijuana at least once in their lifetime, yet less than 4% of the drivers had a permit to purchase medical marijuana. Most of the drivers in the California study first tried marijuana between the ages of 14-17 26.

Marijuana use can impair driving abilities such as reaction time, and lane position 27, specifically marijuana impairs “every performance area that can reasonably be connected with safe driving of a vehicle, such as tracking, motor coordination, visual functions, and particularly complex tasks that require divided attention” 28. In a 2012 systematic review of motor vehicle collisions involving cannabis consumption, researchers determined that driving under the influence of marijuana nearly doubles the risk of being involved in a fatal collision. They also observed that the rates of using marijuana within 1 hour of driving doubled from 1996/1997 to 2004 in Canadian drivers 29.

Therapeutic uses for marijuana:
Marijuana is used to treat glaucoma, to stimulate appetite in anorexia, and in conjunction with cancer chemotherapeutics to reduce nausea and vomiting 30. Michael Bostwick, MD from the Mayo Clinic observed that “Public approval drives medical marijuana legalization efforts without the scientific data normally required to justify a new medication’s introduction.” 9 Several states require medicinal marijuana users to be registered to control who can access the drug, however researchers have shown that medicinal marijuana is diverted from registered users to adolescents for recreational use 31. Getting a permit is quite easy and the vast majority of permit holders in states like California don’t come close to be the type of people the public thinks the law is meant to help (i.e. terminally ill cancer patients).

Regulatory status:
The United States Department of Justice Drug Enforcement Administration (DEA) lists marijuana as a schedule I drug under the Controlled Substances Act (CSA), defined as: “drugs with no currently accepted medical use and a high potential for abuse. Schedule I drugs are the most dangerous drugs of all the drug schedules with potentially severe psychological or physical dependence” [Emphasis added] 32. Twenty three states and the District of Columbia have passed laws allowing for legal use of medical marijuana; Colorado and Washington allow possession for personal, recreational use 33.

Advocates have argued that marijuana use should be legalized, based on selected studies citing limited health and social effects of marijuana use.

Marijuana is the most widely used illicit drug in the United States 34. Support of legalization of medicinal and recreational marijuana is growing 1 and 23 states and the District of Columbia have passed laws allowing medicinal marijuana – largely due to the groundswell of public support. Several negative health effects of marijuana use have been established including short- and long-term neurological effects, addiction, and exposure to carcinogens. Marijuana impairs driver’s abilities and risk of vehicle collision increases with the amount of marijuana consumed 29. Experts have noted that there is a gap in health and safety data of recreational marijuana use that needs to be closed prior to making public policy decisions 35.

1 2013 Gallup survey
2 Morgan (2010) Cannabidiol attenuates the appetitive effects of d9-tetrahydrocannabinol in humans smoking their chosen cannabis. Neuropsychopharmacology.
3 Sullivan (2013) Determination of Pesticide Residues in Cannabis Smoke;

4 Greenson (2013) What are you smoking? Study finds pesticides transfer to marijuana smoke
5 Collins (2013) Conn. University to test pot for contaminants
6 Campaign to Regulate Marijuana Like Alcohol. “Marijuana, Health & Safety” (Accessed 11-19-2013)
7 Drugs of Abuse – a DEA Resource Guide (2011 Edition)
8 Chadwick et al. (2013) Cannabis use during adolescent development: susceptibility to psychiatriric illness. Forteins in Psychiatry (
9 Bostwick (2012). Blurred boundaries: the therapeutics and politics of medical marijuana.
10 Meier, MH, et al (2012) Persistent cannabis users show neuropsychological decline from childhood to midlife. PNAS.
11 Substance Abuse Treatment Admissions by Primary Substance of Abuse, According to Sex, Age Group, Race, and Ethnicity.
12 Facts and Answers to the Frequently Asked Questions about Marijuana
13 Results from the 2011 National Survey on Drug Use and Health: Summary of National Findings (SAMHSA)
14 Addiction Science: from Molecules to Managed Care (2008)
15 Pacula (2010) Examining the Impact of Marijuana Legalization on Harms Associated with Marijuana Use.
16 Marijuana smoke listed effective June 19, 2009 as known to the state of California to cause cancer
17 California Proposition 65 list of chemicals known to the state to cause cancer or reproductive toxicity (November 8, 2013)
18 Evidence on the carcinogenicity of Marijuana smoke, August 2009 (page 11)
19 Hashibe (2005) Epidemiologic review of marijuana use and cancer risk. Alcohol
20 Desbois AC and Cacoub P., Cannabis-associated arterial disease. Ann Vasc Surg. 27(7):996-1005, 2013
21 Thomas G et al., Adverse cardiovascular, cerebrovascular, and peripheral vascular effects of marijuana inhalation: what cardiologists need to know. Am J Cardiol. 113(1):187-90, 2014
22 Hodcroft CJ et al., Cannabis-associated Myocardial Infarction in a Young Man with Normal Coronary Arteries. J Emerg Med. . pii: S0736-4679(13)01403-0, 2014
23 National Survey on Drug Use and Health (2012).
24 NORML “Marijuana and Driving: A Review of the Scientific Evidence” (Accessed November 21, 2013)
25 Adam Nagourney and Rick Lyman (2013) Few Problems With Cannabis for California.
26 California Office of Traffic Safety 2012 Survey
27 Ronen (2008) Effects of THC on driving performance, physiological state and subjective feelings relative to alcohol. Accident Analysis and Prevention.
28 Sewell (2010) The Effect of Cannabis Compared with Alcohol on Driving.
29 Asbridge, Hayden, and Cartwright. 2012. Acute Cannabis consumption and motor vehicle collision risk: systematic review of observational studies and meta-analysis.
30 Volkow ND. (2014) Adverse health effects of marijuana use. N Engl J Med 5;370(23):2219-27.
31 Thurstone (2011) Medical marijuana diversion and associated problems in adolescent substance treatment.
32 US Department of Justice Drug Enforcement Agency Drug Schedules
34 Results from the 2010 National Survey on Drug Use and Health: Summary of National Findings (SAMHSA)
35 Pacula and Sevigny (2013) Marijuana Liberalization Policies: Why We Can’t Learn Much from Policy Still in Motion.

Prepared by the Marijuana Science Forum, Oct. 2014

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