Cannabis and the general practitioner – Going to Pot

Cannabis use is increasing across the population, with evidence that cannabis smoking amongst young people is becoming more common than tobacco use. Genetically modified cannabis produces a significantly more potent drug than 20 years ago, this combined with ingestion techniques designed to increase the concentration of cannabinoids inhaled means that the health risks of cannabis cannot be ignored.

The debate around cannabis has always focused on and been polarised by views on legalisation with the health effects largely un-debated. As general practitioners it is likely that over the years we will encounter greater numbers of heavy, dependent cannabis users, presenting with predominately mental health or respiratory problems. It is important that as a profession we are able to offer evidence based information and advice on how to reduce the harms associated with use.

A literature search for the health risks of cannabis will reveal numerous academic papers and scores of reviews[i] on the subject yet few general practitioners and fewer still of the public can cite one problem with the commonest illicit substance used today. That cannabis contains at least as many carcinogens as tobacco smoke, that it is used in a way that maximises its harmful effects and that it more than likely contributes to the production of psychosis in young adults may surprise many readers, who like most of the population think that the drug is at worst harmless and at best good for you. It is a credit to the pro-cannabis debate that the legalisation issue has proved such an effective ‘smoke screen’ to health risks associated with the drug.

It is hardly surprising that cannabis has health risks. It contains over 400 chemicals[ii] including 60 or so carcinogens[iii] and is a markedly stronger drug than 20–30 years ago. A ‘reefer’ in the 1980s contained about 10mg of 9-tetrahydrocannabinol (THC), the main active chemical, where as a ‘joint’ today may contain around 300mg THC[iv]. Most of the effects of cannabis are dose related and hence this change in strength is important in determining its health effects.

So what are the health risks? These are considerable and as the drug becomes more widespread they are becoming better defined. The known effects of cannabis are analogous to the known effects of alcohol and tobacco, though its dangers are less obvious. The evidence that it produces dependence is beyond dispute, with around 5-10% eventually becoming dependent[v]. Cannabis also impairs concentration, short-term memory, attention and rational thought, [vi] impairs driving and piloting skills and amplifies the driving impairments caused by concomitant alcohol use[vii]. Larger amounts of cannabis can produce anxiety and depression[viii], psychotic states lasting several days and an increased risk of developing schizophrenia[ix].

Most of the ill effects of cannabis are related to smoking the drug, the most popular route of use, usually in the resin form mixed with tobacco. The British Lung Foundation[x] in their review of the respiratory effects of cannabis highlight a number of important factors that make cannabis cigarettes at least as harmful as tobacco ones. Studies estimate that three to four cannabis cigarettes per day are associated with the same evidence of acute and chronic bronchitis and the same degree of damage to the bronchial mucous membrane as 20 or more tobacco cigarettes a day.

Tar from cannabis cigarettes contains up to 50% higher concentrations of the carcinogens than tobacco smoke. Users smoke cannabis cigarettes in a manner that maximises the concentration of these chemicals at the bronchial mucosa. Compared to cigarette smokers, cannabis smokers take deeper, longer breaths and retain the smoke for longer periods. The habit of smoking the cigarette down to the butt deposits four times as much tar on the respiratory tract in comparison to unfiltered tobacco cigarettes, hence amplifying the exposure of cannabis smokers to particles that are known to be involved in the development of lung cancer.

Possible therapeutic effects

Cannabis has been widely used as a medicine for over a millenium in China, India and the Middle East and other parts of the World. There is evidence that it is effective against chemotherapy induced nausea and vomiting[xi] and as an analgesic[xii] and that it is valuable for combating the loss of appetite and weight loss experienced in patients with cancer and AIDS, for the relief of muscle spasms in patients with multiple sclerosis[xiii] and in the treatment of glaucoma. Randomised controlled trials designed to compare cannabis with existing treatment will show if cannabis proves superior to existing remedies and, if so, there should be no bar to it being made available to appropriate patients in the appropriate formulation. However, any claims for therapeutic effects must be backed up by robust evidence and the benefits outweigh the risks[xiv].


The reputation that cannabis is a safe drug is unjustified. The reasons for this misplaced view are that it is not immediately lethal in the way that heroin is, that its effects on mood state are not as obvious as alcohol and that its capacity to produce dependence, like alcohol is slow and insidious and its widespread use is a relatively new phenomena. Recent Inquires [xv][xvi][xvii]on cannabis have come to the same conclusion, that cannabis can be harmful and that its use should be discouraged. Although cannabis is not in the premier league of dangerous substances, new research tends to suggest that it may be more hazardous to health than might have been thought only a few years ago. Smoking cigarettes now accounts for an excess of 35,000 deaths from lung cancer, 30,000 deaths from chronic lung disease and 35,000 deaths from other causes. If the prevalence of cannabis use were to approach those of alcohol or tobacco then its public health impact would also increase and would lead to an increase in psychological and physical morbidity.

What can doctors do?

Faced with the current and probable increased level in the use of cannabis is there anything a doctor can do? Certainly there is an important role in providing accurate information to users (and potential users) and ways, if possible, of reducing their risks. However, perhaps the greatest impact that doctors can have is by influencing the political landscape surrounding the use of cannabis. Perhaps we need to learn from the decades of cigarette smoking. During the tobacco industry’s heyday, when cigarettes were an accepted part of life, doctors, dentists and nurses were lighting up on the advertising pages of leading newspapers and magazines to promote the product, the adverts part of the industry’s efforts to convince people that smoking was not just acceptable but ‘healthful’.[xviii] This state of affairs changed when the link between smoking and lung cancer was established. Now few doctors smoke and they lead the way in anti-smoking campaigns and ensuring that the harms caused by the drug are well disseminated[xix]. Perhaps it is this role, as promoters of political change, that doctors must now take with cannabis. To effect change doctors themselves must be convinced of the dangers to the publics health of the drug and beware of entering the polarised and unhelpful debate over legalisation.

There is evidence emerging that cannabis smoking among the young is now becoming more common than tobacco smoking[xx] let us be prepared.

This position statement was prepared by Dr Clare Gerada, former-Project Director of the Royal College of General Practitioners Drug Training Programme and endorsed by UK Council on 12 September 2003.

Summary of adverse effects of cannabis [xxi]

Acute effects

  • Anxiety and panic, especially in naive users.
  • Impaired attention, memory, and psychomotor performance while intoxicated.
  • Possibly an increased risk of accident if a person drives a motor vehicle while intoxicated with cannabis, especially if used with alcohol and tranquillisers.
  • Increased risk of psychotic symptoms among those who are vulnerable because of personal or family history of psychosis.
  • Increase risk of low birth weight babies if smoked in pregnancy.

Chronic effects (uncertain but most probable)

  • Chronic bronchitis and histopathological changes that may be precursors of malignant disease.
  • A cannabis dependence syndrome characterised by an inability to abstain from or to control cannabis use, craving and tolerance to the physical and mental effects of the drug. The risk of dependence is estimated at less than 20% if the drug is taken more than five times and 10% for those who have ever used. Between 15-30% of users report difficulty in controlling their use and withdrawal symptoms are common in this group.
  • Subtle impairments of attention and memory that persist while the user remains chronically. intoxicated, and this may or may not be reversible after prolonged abstinence .

Possible adverse effects (to be confirmed)

  • Increased risks of cancers of the oral cavity, pharynx and oesophagus: leukaemia among the offspring exposed in utero.
  • Impaired educational attainment in adolescents and under-achievement in adults in occupations that require high-level cognitive skills.
  • Groups that are at higher risk of experiencing these adverse effects:
  • Adolescents with a history of poor school performance, who initiate cannabis use in their early teens, are at increased risk of using other drugs and of becoming dependent on cannabis.
  • Women who continue to smoke cannabis during pregnancy may increase their risk of low birth weight babies.
  • People with schizophrenia, asthma, bronchitis alcohol and other drug dependence, whose illnesses are exacerbated by cannabis use.

Summary of effects in non-vulnerable individuals

  • Panic attacks
  • Toxic psychosis
  • Summary of effects in vulnerable individuals
  • Individuals with mental disorder are at increased risk of misusing cannabis.
  • There is 4–6 times increase risk of cannabis misuse in schizophrenia.
  • Heavy use of cannabis is a probable risk factor for schizophrenia.
  • Cannabis use may worsen some symptoms of schizophrenia and increase the rate of relapse.
  • Heavy cannabis use may cause short term reversible cognitive impairments in adolescents  

[i]World Health Organisation. 1997 Programme on substance abuse: cannabis: a health perspective and research agenda. Geneva: WHO

[ii] Maykut MO (1985) Health Consequences of acute and chronic marihuana use. Prog. Neuropsychopharmacol. Biol. Psychiatry 9:209-238

[iii]Lishman WA 1998 Organic Psychiatry: The Psychological Consequences of Cerebral Disorder. (3rd Edition), Blackwell, Oxford

[iv]Gold MS (1991) Marijuana, In Comprehensive Handbook of Alcohol and Drug Addiction. Marcel Dekker Inc, New York, pp 353 - 376

[v]Swift W, Hall W and M Tesson 2001 Cannabis use and dependence among young Australian adults: results from the National Survey of Mental State and Wellbeing. Addiction 96; 737-748

[vi]Chait LD., Pierri J (1992) Effects of smoked marijuana on human performance: A critical review. In Marijuana / Cannabinoids: Neurobiology and Neurophysiology (Eds L Murphy &A Bartke) CRC Press, Boca Raton pp 387 - 424

[vii]Ashton CH. Adverse effects of cannabis and cannabinoids. British Journal of Anaesthesia 1999; 83: 637-49

[viii]Patton G., Coffey C Cannabis use and mental health in young people: cohort study. BMJ 2002 325:1195-1198

[ix]Zammit S., Allebeck P., Andreasson S et al. Self reported cannabis use as a risk factors for schizophrenia in Swedish conscripts of 1969: historical cohort study. BMJ 2002: 325:1199-1201

[x]British Lung Foundation. A smoking gun? The Impact of Cannabis Smoking On Respiratory Health. Published 11th November 2002. 78 Hatton Garden, London EC1N 8LD

[xi]Tramer MR, Carroll D, Campbell FA et al Cannabinoids for the control of chemotherapy induced nausea and vomiting: a systematic review. British Medical Journal. 2001; 323 (7303): 16- 21

[xii]Tramer M, Carroll D, Campbell FA, et al. Are cannabinoids an effective and safe treatment option in the management of pain? A qualitative systematic review. British Medical Journal 2001; 323 (7303):13-16

[xiii]Cannabis Trail launched in patients with MS. British Medical Journal. 2001. 322(7280):192

[xiv]Robson P. Therapeutic aspects of cannabis and cannabinoids. British Journal of Psychiatry. 2001 Feb; 178: 107-15

[xv]Runciman R. Drugs and the Law. Report of the Independent Inquiry into the Misuse of Drug Act 1971.The Police Foundation. 2000:

[xvi]Select Committee on Home Affairs Third Report 2002

[xvii]House of Lords Select Committee on Science and Technology. Cannabis: the scientific and medical evidence. London: Stationery Office, 1988

[xviii]Mahaney F. Old-time Ads tout health benefits of smoking: Tobacco industry had doctors’ help. Journal of the National Cancer Institute. 1994 86:14; July 20.

[xix]Action on Smoking and Health [UK]

[xx]Miller P, Plant M. Heavy Cannabis use among UK teenagers: an exploration. Drug Alcohol Depend 2002: 65: 235-242

[xxi]Hall W., Solowij N. Lemon J (Eds) 1994 The Health and Psychological Consequences of Cannabis Use. National Drug Strategy Monograph Series No 25, Canberra, Australian Government Publishing Service.

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