Summary of the effects of marijuana use

Marijuana (public domain)

By Spencer D Gear

1.    One cigarette (joint) impairs the short term memory for at least 6 weeks.  There are many studies demonstrating the deterioration of short term memory in marijuana users.  The definitive and best controlled of these was done in 1989 by Dr Richard Schwartz.  He demonstrated persisting impairment of short term memory six weeks after supervised abstention from the drug.  Just one joint is all that is needed.  (Dr’s Richard Schwartz, Gruenewald, M Klitzner et al “Memory Impairment In Cannabis Dependent Adolescents”, Am, J. Dis, Child, 143:1214-19, 1989 – Georgetown Medical School – Washington DC).  Take a read of this one from The New Scientist, “Natural high helps banish bad times.”

2.    In a major study to investigate the effects of cannabis on motor skills, twenty four hours after one cigarette (joint), experienced pilots performed severely impaired simulator landings.  These pilots reported that they felt absolutely fine, with normal mood, alertness and performance and were completely unaware of their impairment.  Several major rail crashes in     the USA have been associated with the use of marijuana.  (Dr JA Yesavage, VO Leirer, DG Morrow, Stanford University – “Marijuana carry over effects on aircraft pilot performance” – Aviation, Space and Environmental Medicine, 62:221-27, 1991)  Marijuana use is a continuing concern to paediatricians.

What about road accidents?

Cannabis and driving: a new perspective” by Carl J O’Kane, Douglas C Tutt and Lyndon A Bauer, warns of the influence of marijuana use on one’s ability when driving a motor vehicle [Emergency Medicine, Volume 14 Issue 3 Page 296  – September 2002].  Whilst much research exists from overseas relating to increased risk of motor vehicle accidents due to marijuana use, the following Australian data are significant.

Dr Judith Perl, pharmacologist, of the Clinical Forensic Medicine Unit – NSW Police Service released information in 1991 of a study conducted over the period 1987-90.  The study involved taking blood and urine samples from accident victims in four Sydney hospitals at random.  The only qualifier was that those measuring .05 BAC [blood-alcohol concentration], or known to have consumed alcohol were not tested for other drug use.  The increase in positive testing for cannabis in the blood of these victims was staggering, increasing from 28% (87-88) to 68% (1990).  [See also Judith Perl,  “Drugs & traffic safety”, Australian Journal of Forensic Sciences 17:25]Mrs Kate Carnell stated in “Debates of the Legislative Assembly for the Australian Capital Territory” (Hansard, 9 September 1992, p. 2077) that:

“Cannabis is clearly a cause of driver impairment – a fact of which we are becoming incresasingly aware.  A study conducted by Dr. Judith Perl, of the forensic unit of the New South Wales Police, shows that cannabis is the single most important source of driver impairment discovered in blood and urine samples.  Cannabis constituted 68 per cent of all drug-positive urine and blood tests conducted in New South Wales during 1990.  Thus the threat that cannabis poses to driving safety is not idle and it must not be ignored.  We know that alcohol also affects driving ability, judgment and skill performance, but the residual effects of cannabis last much longer than those of alcohol.”

3.    A 15 year research project at the Karolinska Institute and Juddinge University Hospital, Sweden, revealed a 600% increase in the incidence of schizophrenia in conscripts who had used marijuana 50 times or more in their lifetime.  This study used a standardised method for the diagnosis of schizophrenia.  (Longitudinal study at the Karolinska Institute in Sweden – 15 year study using 45570 army conscripts – Dr Sven Andreasson,  P Allerbeck, A Engstrom et al., Cannabis and Schizophrenia: A Longitudinal Study of Swedish Conscripts.  The Lancet, 2:1483 -1485,1987).

4.    A parallel study showed a 500% increase in the overall incidence of other psychiatric disorders in conscripts who were users. (Andreasson, S; Allerbeck, P; Rydberg, U., “Schizophrenia in Users and Non Users of Cannabis”  Acta Psychiatr. Scan., 79:505-510, 1989)  The use of cannabis in adolescence and risk for adult psychosis was examined in a New Zealand: longitudinal prospective study.  It found that “early cannabis use (by age 15) confers greater risk for schizophrenia outcomes than later cannabis use (by age 18). The youngest cannabis users may be most at risk because their cannabis use becomes longstanding.” [BMJ BMJ. 2002 November 23; 325 (7374): 1212–1213] [2].  The New Scientist reports on another study confirming the “Cannabis link to mental illness strengthened“.

5.    The Swedish study scientifically linked marijuana to the dramatic increase in drug-induced schizophrenoform illness and the associated increase in teenage suicide rates and other violent death (as above )

6.    The so called “Amotivational syndrome” –
Portal icon  Apathy, poor judgement, lack of self care,
Portal icon  Decreased empathy (perception of others problems)
Portal icon  Impaired perception of past, present and future.
Portal icon  Difficulty with information processing.
Portal icon  Difficulty with sequential dialogue.
(Goodman & Gilman – “The Pharmacological Basis of Therapeutics” 8th Ed. 1991)  For the latest edition.

Dr Robert C Gilkeson, – a teacher, paediatrician, adolescent neuropsychiatrist and brain researcher, specialising in early childhood development and learning disabilities, moved in 1987 (after some years of paediatric practice and consultancy) to devote his time to research the effects of marijuana on brain function.  Up until his untimely death in 1993, he was the Director of the Center for Drug Education and Brain Research.  He summarised his general findings in a paper to the US Committees of Correspondence, Drug Watch division with this quote:

“My research studies of youngsters from kindergarten through high school show previously well adjusted and intellectually endowed children falling apart academically and emotionally in the teenage years with the only new factor being that of occasional marijuana use.  Marijuana use can lead to an inability to retain strong self image, and an inability to visualise and plan for the future.  Using marijuana makes ‘great’ people feel average, and ‘average’ people ‘dumb’.  Marijuana use is toxic to all cells, and most especially toxic to brain cells.

“In 1981, my eight year study of 90 adolescent marijuana smokers was completed.  Each youths brain wave tracings (EEG) showed dysfunction (decreased activity) similar to brain wave tracings of the learning disabled.  A decrease in brain cell energy causes a decline in the level and complexity of thought and behaviour.  ‘Burned out’ kids with impairment to both their frontal lobe and their short term memory due to chronic intoxification of marijuana were evident.

“These impairments are the cause of the increased violent and non violent juvenile crime, truancy and school drop out, teenage runaways and vagrancy, teenage prostitution and pregnancy, venereal disease, adolescent depression and suicide, polysubstance use and adolescent psychiatric referrals.  Most alarming of all is the fact that we have witnessed the appearance of a new chronic organic brain syndrome called ‘burnout’ caused by marijuana use.”

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Recent research in animals has also suggested that long-term use of marijuana (THC) produces changes in the limbic system that are similar to those that occur after long-term use of other major drugs of abuse such as cocaine, heroin, and alcohol. These changes are most evident during withdrawal from THC. During withdrawal, there are increases in both the levels of a brain chemical involved in stress and certain emotions and the activity of neurons in the amygdala. These same kinds of changes also occur during withdrawal from other drugs of abuse, suggesting that there may be a common factor in the development of drug dependence (Connecticut Clearinghouse, “Marijuana: The Brain’s Response to Drugs,” 1999).

In 1992, a study assessed the acute effects of cannabis on human cognition.  This study found that cannabis impaired all capabilities of learning including associated processes and psychomotor performance.  (Block RI, Farinpour R & Braverman K., “Acute effects of marijuana on cognition: relationship to chronic effects and smoking techniques. Pharmacology Biochemistry and Behaviour,” 1992, 43(3):907-917).  Here is a summary of that research.  Also take a look at: “Marijuana use during pregnancy damages kid’s learning” (The New Scientist).

“Much recent research is showing us exactly how marijuana impairs the brain. For instance, three days or more after smoking marijuana, PET scans of chronic marijuana users show decreased metabolic activity in the brain, especially in the cerebellum, a part of the brain involved with motor coordination, learning, and memory [Volkow ND et al., Psychiatric Research Neuroimaging 67:29-38, 1996]” (quoted from, “Prof. Miron Is Wrong About Marijuana,” Janet D. Lapey, M.D., The Massachusetts News Columnist, February 2000).  However, The New Scientist claims that “Controversy still rages over whether cannabis damages the brain.”

For a summary of information for teenagers see:  “Tips for Teens: The Truth About Marijuana.” 

7.    Four times the cancer causing potential of cigarettes.  Cancers of the mouth and jaw usually seen in men (over 60 ) who had been heavy smokers and drinkers for decades have been found in young users.  All had been daily marijuana users but had not smoked nicotine and only used a small amount of alcohol if any.  Study group was young men between 19-38 who had developed squamous cell cancers of the tongue or jaw with lymph node involvement. (PJ Donald – “Marijuana Smoking – Possible Causes of Head and Neck Carcinoma in Young Patients” Otolaryngology Head and Neck Surgery, 94:517-521, 1986 – University of California, and Hoffman, D.; Brunnermann, KD.; Gori, GB.; Wynder, EL., “On the Carcinogenicity of Marijuana Smoke”.  In: Runeckles, VC., ed. Recent Advances in Phytochemistry, New York: Plenium, 1975:63-81.) 

The New Scientist reports on “Cannabis smoking ‘more harmful’ than tobacco.”

“Marijuanasmoking is associated with a dose-dependent increased risk ofhead and neck cancer. . .  Marijuana is a riskfactor for human head and neck cancer ” (“Marijuana Use and Increased Risk Zuo-Feng Zhang, Hal Morgenstern, Margaret R. Spitz, Donald P. Tashkin, Guo-Pei Yu, James R. Marshall, T. C. Hsu and Stimson P. Schantz (Cancer Epidemiology Biomarkers & Prevention Vol. 8, 1071-1078, December 1999)

Here’s a summary of risk factors for head and neck cancer, including the use of marijuana.

Although scientists have been convinced in the past that smoking causes lung cancer, the strong statistical associations did not provide absolute proof. This paper absolutely pinpoints that mutations in lung cancer cells are caused by benzopyrene. An average marijuana cigarette contains 30 nanograms of this carcinogen compared to 21 nanograms in an average tobacco cigarette (Marijuana and Health, National Academy of Sciences, Institute of Medicine report, 1982). This potent carcinogen suppresses a gene that controls growth of cells. When this gene is damaged the body becomes more susceptible to cancer. This gene, P53, is related to half of all human cancers and as many as 70% of lung cancers.

Commentary: Clearly marijuana smoke contains more of the potent carcinogen benzopyrene than tobacco smoke. Furthermore, the technique of smoking marijuana by inhaling deeply and holding the smoke within the lungs presents a chance of much greater exposure than a conventional tobacco cigarette. (Commentary provided by William M. Bennett M.D., Professor of Medicine, Division of Nephrology, Clinical Pharmacology and Hypertension at Oregon Health Sciences University, Portland, Oregon.  This information is from Drug Watch Oregon).

8.  Depression of the immune system at both humoural (body fluids) and cell immunity levels. In fact the immune system response is lowered by up to 40%.  Studies have shown for instance that young people who are users tend to be ill more frequently than non users.  Dr Akira Morishima has found that marijuana more than any other drug he had studied is closely     correlated with a high rate of chromosome damage or destruction particularly in relation to T- lymphocytes (white blood cells). [Friedman, H; Klein, TW; Newton, CA; Widen, R., “The Effects of Delta-9-tetrahydrocannabinol and 11-hydroxy-delta-9-tetrahydrocannabinol on 7-lymphocyte and B-lymphocyte Mitogen Response”. J. Immunopharmacol., 7,451,1985 Florida University – 1985 &1994 Drugs of Abuse and the Immune System; 1st International Symposium Paris 1990  & A Morishima, GG Nahas & et al].

“There is good evidence that THC and other cannabinoids can impair both cell-mediated and humoral immune system functioning, leading to decreased resistance to infection by viruses and bacteria. However, the health relevance of these findings to human marijuana use remains uncertain. Conclusive evidence for increased malignancy, or enhanced acquisition of HIV, or the development of AIDS, has not been associated with marijuana use” (National Institutes of Health – Workshop on the Medical Utility of Marijuana, February 19-20, 1997)

For a contrary opinion, see “Marijuana and Immunity,” Leo E. Hollister M.D. (Journal of Psychoactive Drugs pp. 159-163 Vol. 24 Apr-Jun 1992).

Portal icon9.    Fertility and other sexual development problems in males and females.

Males: sperm production is reduced, sperm motility reduced, production of testosterone and other hormones are reduced or delayed, which inhibits normal sexual development in males.  Studies indicate that sometimes this sexual developmental delay leads to lack of interest in females and normal copulatory behaviour.  Another side effect is the chromosomal damage (up to three times the normal rate) giving rise to the inability to produce normal pregnancy.

Females:  marijuana can cause defective menstrual cycles, damage the ovum, cause production of high levels of testosterone, and significantly reduce levels of prolactin, which is required for milk production.  Additionally females who use during pregnancy or who have residual levels of THC still present in their bodies are shown to produce lower than normal birthweight babies and, especially males with a higher than normal mortality rate. (Dr Wylie Hambree et al Columbia University; Dr Susan Dalterio University of Texas; Mendelsen JH et al Journal of Pharmacology & Experimental Therapeutics, 1978, 207:611-617; Dr Ethel Sassanrath, University of California; Hingson et al ‘Paediatrics’, vol 70 Oct 92 – Marijuana Alert.  Hatch, E; Bracken, M., “Effect of Marijuana Use on Foetal Growth.”  Am. J. Epidemiol. 124, 986, 1986.  Fried, P; Watkinson, B; Willan, A., “Marijuana Use in Pregnancy and Decreased Length of Gestation.”   Am. J. Obstet. Gynecol., 105, 23, 1984)

A new study at the University of Buffalo, USA, has found: “Men who smoke marijuana frequently have significantly less seminal fluid, a lower total sperm count and their sperm behave abnormally, all of which may affect fertility adversely, a new study in reproductive physiology at the University of Buffalo has shown” (University of Buffalo Reporter, October 23, 2003).

Researcher Peter Fried, a psychologist at Carleton University in Ottawa, Canada, “told New Scientist (25 March 2003) that as well as affecting memory and learning, exposure to marijuana during pregnancy has a strong effect on visual mapping and analysis in human children.”

10.    DNA metabolism is inhibited thus interfering with cell function and replication.  The blockage of this process results in slowing down the manufacture of DNA, RNA and proteins in the cell nucleus – a process essential for cell life.  (B. Desoize; G Nahas; C Latour; R Vistelle,  University of Champagne – Ardenne, “In Vivo Inhibition of Enterocyte Metabolism by Delta-9-THC” Pro. Soc. Exp. Biol. Med., 181, pp. 512-516, 1986)

11.    Associated with the above the THC enlarges the area between each cell, resulting in poor transmission of nerve impulses.  This can lead to impaired speech and comprehension of complex ideas, loss of memory, difficulty in concentrating, insomnia, lack of body coordination and loss of muscle strength, impaired vision and unexpected mood changes.  (RG Heath et al – “Chronic Marijuana Smoking  – its effect on the Function and Structure of the Primate Brain”.)
Again associated with the issue of DNA, RNA, cell function and replication is the issue of birth abnormalities being produced in the offspring of parents who have used or are still using marijuana.  These abnormalities closely resemble those of thalidomide babies although where thalidomide produced such abnormalities called phecomelia – in place of hands and feet, new borns had seal-like flippers.

Marijuana is responsible for defects such as non-existent limbs, phocomelia, syndactyly (fingers are fused together rendering them useless), missing hands and forearms, webbing of the hands, lack of nails, club feet and hydrocephalus (so called water on the brain).  Dr Virchel E Wood, Orthopedic Surgeon & Associate Professor of the School of Medicine – Dept of Orthopedic Surgery – Loma Linda University (USA) has indicated that abnormalities can occur in the young of one or both parents who have been shown to have used marijuana.  People who use marijuana and other drugs have 18 times more birth defects than non users.

n research reported in 2003, Drs Kenneth L. Audus, and Michael J. Soares of the Institute of Maternal-Fetal Biology concluded that “illicit drugs (e.g. cocaine, marijuana, etc) taken by the mother at virtually any time during gestation have the potential to adversely affect the outcome of pregnancy, resulting in severe complications for the mother, pre-term birth, abnormalities in fetal development and increased health risks as the newborn grows into adulthood” [” Dr. Audus is an internationally recognized expert on drug metabolism and drug transport by the placenta, while Dr. Soares’ expertise resides in understanding mechanisms controlling the growth and development of the placenta”] (News Release, September 1, 2003).

Dr Susan Dalterio of the University of Texas (San Antonio) has noted in extensive studies that genetic mutations have passed through to the second generation of offspring of marijuana users.


Such warnings [about marijuana use linked to psychoses] should not surprise the scientists who have for many years maintained that the THC contained in marijuana is dangerous. First, in the late 1960?s Dr. Robert Heath, then chairman of the Department of Psychiatry and Neurology at Tulane Medical School, found that marijuana affects brain waves and destroys brain cells. [3] Second, a study conducted by Dr. Ethel Sassenrath at the University of California at Davis between 1974 and 1978 found that THC increased the rate of fetal loss (in utero, fetal death) in monkeys by over 300%, while at the same time decreasing the birth weights in those babies born alive. [4] Third, a study by Dr. Susan Dalterio, at the University of Texas found that marijuana decreased testosterone and impaired sexual development in male mice. [5] Finally, a study by Dr. Albert Munson found that injections of THC suppressed the immune systems of mice and made them 96 times more susceptible to the herpes virus. [6] (Schaffer Library of Drug Policy)

12.    1100% increase in the incidence of acute non lymphoblastic leukaemia in the offspring of mothers who used while pregnant or just prior to conception.  The research also indicated that that these children developed the leukaemia earlier – 19 months instead of the usual 93 months.  (Professor Neglia et al Minnesota University – reported 1990 and Robson et al Children’s Cancer Study Group – reported in “Cancer” 63:1904-1910, 1989)

13.    Marijuana prevents liver enzyme CP450 from breaking down anti-depressant medication thus causing an accumulation of the anti-depressant in the body which can result in death (Dr John Anderson – Neuro Scientist, Consultant, Psychophysiologist – Neuroscience Psychological Services Centre,  Westmead,  Sydney NSW).  It is tragic for the scientific cause of the investigation of the impact of marijuana and anti-depressants that Dr. Anderson died in 2002.

  I would like to see in-depth research conducted to follow-up Dr. Anderson’s pioneering work.  Here is a summary of Dr. John Anderson’s preliminary research.  Further, Dr. Anderson contended:

Statistics suggest that 40% of ADHD children are predisposed to substance abuse during adolescence or adulthood. Of the ADHD population who are poly substance users, 67% smoke marijuana. Many behavioural changes are similar to those of ADHD: academic ability decreases; sniffles, colds, trivial illness, especially respiratory system; concentration levels decrease; depersonalisation; increased levels of anxiety; increased depression; reaction times slows; short-term memory difficulties; a lack of motivation or interest in things previously enjoyed; increased impulsivity; space and time distortion; may increase appetite.  (A summary of a talk presented by John Anderson to ADDult, NSW, Australia)

14.    Marijuana use and its link to other illicit drugs, is not genetic according to
Michael Lynskey, at Washington University School of Medicine in St Louis, Missouri, and his team [who] found that the early user [of marijuana] was two to five times more likely to go on to use harder drugs or become dependent on alcohol – regardless of whether they were an identical twin or not.

    The fact that identical twins, who share all their genes, did not differ from non-identical twins, who share half, suggests that the progression is not the product of genes. (The New Scientist, 21 January 2003, based on an article in the  Journal of the American Medical Association, vol 289, pp. 427, 482).

15.  Yet, there is a rising swell of support for marijuana use across Europe and Canada, according to The New Scientist

For further marijuana research summaries, see the Drug Watch Oregon website.  See “Marijuana: Facts for Teens.”
Here’s a short summary of the effects of marijuana (a summary of this article).


  • See the Washington State University article, “Marijuana Effects” that confirms the deleterious consequences of marijuana use.


1.  Susan Dalterio is a Senior Lecturer in the Department of Biology at the University of Texas at San Antonio.  Her email contact is:  [email protected]

2.  Copyright © 2002, BMJ BMJ. 2002 November 23; 325 (7374): 1212–1213, “Cannabis use in adolescence and risk for adult psychosis: longitudinal prospective study, ” Louise Arseneault, lecturer, Mary Cannon, Wellcome Trust advanced fellow, Richie Poulton, director, Dunedin multidisciplinary health and development study, Robin Murray, professor, Avshalom Caspi, professor, Terrie E Moffitt, professor. 

SGDP Research Centre, King’s College, London SE5 8AF, Division of Psychological Medicine, King’s College, Dunedin Multidisciplinary Health and Development Research Unit, University of Otago, Dunedin, New Zealand. Correspondence to: T E Moffitt [email protected]

3.  Robert G. Health, “Cannabis Sativa: Effects on Brain Function,” Biological Psychiatry, Vol. 15, No. 5, 1980.

4. Government’s Supplemental Sentencing Memorandum Re: Health Effects of Marijuana, U.S. v. Greyshock, United States District Court for the District of Hawaii, 1988.

5.  Ibid.

6.  Ibid.


Copyright © 2012Spencer D. Gear. This document last updated at Date: 16 October 2015.