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This article is for Medical Professionals

Professional Reference articles are designed for health professionals to use. They are written by UK doctors and based on research evidence, UK and European Guidelines. You may find the Recreational Drugs article more useful, or one of our other health articles.

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Synonyms (street names): hash, hashish, weed, pot, marijuana, ganja, dope, skunk, grass, puff

  • Cannabis is from the flowering plant Cannabis sativa L. - subspecies sativa.
  • Cannabis is a drug that produces euphoria and reduces anxiety.
  • It can cause dependence; however, addiction, as shown by increasing use and inability to discontinue use, is uncommon.
  • The drug can be used occasionally without causing significant social or mental problems. Withdrawal may cause anxiety and disturbed sleep in heavy users.
  • Cannabis is associated rarely and unpredictably with a range of psychotic symptoms.
  • Cannabis was reclassified from a class C to class B drug in January 2009[1].
  • Cannabis can be smoked or ingested - eg, herbal tea, butter paste.
  • When cannabis is ingested it produces cannabinoids which results in its psychological and physical effects.
  • The most active of these is tetrahydrocannabinol (THC).

According to Home Office Statistics on drug use in the UK from 2016, cannabis continues to be the most commonly used illegal drug, with 6.5% of adults aged 16-59 years having used it in the preceding year (around 2.1 million people) - similar to 2014/15 but statistically significantly lower than a decade ago (8.7%)[2]. Among younger adults aged 16-24 years, cannabis was also the most commonly used drug, with 15.8% having used it in the preceding year (around 975,000 young adults)[1]

Cannabis induces a relaxed state characterised by a series of disconnected thoughts. There may also be:

Psychological effects

  • Relaxation.
  • Euphoria.
  • Alteration in perception of time, colour and space.
  • Short-term memory loss.
  • Irritability.

Physical effects

  • Dry mouth.
  • Dry eyes.
  • Bloodshot eyes.
  • Increased heart rate.

Effects are prolonged for 2-3 hours after smoking, with no clear evidence of hangover or lasting effect.

Ability to drive and ability to operate machinery are impaired due to effects on motor skills and depth perception.

Symptoms of schizophrenia may be worsened.


  • Cannabis use is associated with an increased risk of developing schizophrenia in young people[5].
  • Early cannabis use (<15 years) brings greater risk for schizophrenia than use before the age of 18 years[6].
  • Risk is specific to cannabis and not other drugs.
  • Although most young people use cannabis without problems, a minority of young people experience harmful results. 1 in 10 young people who used cannabis by the age of 15 years developed schizophreniform disorder by the age of 26 years compared with 3% of the rest[7].
  • Cannabis may also be associated with a short-lived psychosis but this may simply represent the initial signs of schizophrenia[8].


  • Use of cannabis in adolescence increases the risk of developing depression and/or anxiety later in life.


  • For some years the general feeling was that cannabis does not produce dependence.
  • However, dependence is common and difficult to treat[9].
  • Animals exhibit a withdrawal syndrome and a similar problem is being recognised in humans[10].

Respiratory function[11]

Regular smoking of cannabis causes visible and microscopic injury to the large airways. This is associated with an increased likelihood of symptoms of chronic bronchitis that subside after stopping use. However, no link with chronic obstructive pulmonary disease (COPD) has been demonstrated. There is no apparent association between light or moderate use and lung or upper respiratory cancer, although insufficient data are currently available to draw conclusions about heavy use. Overall, the risks to the respiratory system are far lower from even heavy regular smoking of cannabis than from tobacco.

Cognitive function[12]

A recent study suggests that cannabis can cause cognitive impairment but that this can be reversed by sustained abstinence.

There have been studies looking at the beneficial effects of cannabis in cancer, AIDS, depression, nausea when using chemotherapy and various neurological diseases - eg, multiple sclerosis (MS). A Scottish study looking at long-term safety was reassuring[13].

  • The only cannabis-related preparation currently available for legal medical use is Sativex®[14].
  • The Medicines and Healthcare products Regulatory Agency (MHRA) granted GW Pharma a licence for Sativex® on 16th June 2010[15].
  • Sativex® is a buccal spray containing the active ingredients delta-9-THC 27 mg/ml and cannabidiol 25 mg/ml, which are both extracts of the hemp plant Cannabis sativa L.


  • Sativex® is indicated for the relief of moderate-to-severe spasticity in adult patients with MS who have not responded adequately to other anti-spasticity medication and who demonstrate improvement to a trial of therapy.
  • Other therapeutic uses of Sativex® which have been explored include:
    • Non-MS neuropathic pain[17].
    • Cancer pain[18].
    • MS bladder dysfunction[19].
  • Areas identified for further development include:
    • Spinal cord injury[20].
    • Rheumatoid arthritis[21].
    • Peripheral neuropathy[22].
    • Cancer care[23].


  • Known or suspected allergy to cannabinoids or other constituents of Sativex® (propylene glycol, ethanol or peppermint oil).
  • Known or suspected history or family history of schizophrenia, or other psychotic illness. History of severe personality disorder or other significant psychiatric disorder other than depression associated with MS.
  • Lactation - a considerable level of cannabinoids can be passed to the infant and adverse effects are possible.

Warnings and special precautions[16]

  • It is not recommended in patients below the age of 18 years.
  • Warn patients they may have 'intoxication-type reactions' - eg, sudden changes in mood, alteration in perception of reality and time and lack of inhibition. These effects are dose-related.
  • Whilst they are taking Sativex®, patients should be warned:
    • Not to drive.
    • Not to operate machinery.
    • Not to do any activity which involves judgement or co-ordination.
  • Use with caution in patients with pre-existing heart disease, arrhythmias, cardiac failure, poorly controlled hypertension, bradycardia, tachycardia and postural hypotension. Avoid in patients with serious cardiovascular disease - eg, coronary heart disease, significant arrhythmias, unstable hypertension, severe cardiac failure.
  • Epilepsy or recurrent seizures - caution is recommended, pending further information.
  • The CNS effects can increase the risk of falls and cause other personal safety problems (eg, pouring hot drinks) particularly in the elderly.
  • Patients with significant hepatic disease or renal dysfunction will have an exaggerated response to Sativex® and should be monitored regularly. Sativex® contains ethanol but 12 sprays a day contain less than 0.5 g.
  • History of substance abuse including alcohol abuse - Sativex® has addictive potential and should be avoided.
  • Spermatogenesis can be affected. Male and female patients should maintain reliable contraception for the duration of therapy and for three months afterwards.
  • Sativex® should not be used in pregnancy (insufficient safety data) unless the benefits to the fetus outweigh the risks.
  • Advise people going abroad to check the legal status of Sativex® in the country to which they are travelling.

Dosage and administration[16]

  • Patients should start with one spray every four hours up to a maximum of four sprays a day and then self-titrate according to response.
  • Warn the patient that it can take a week or more to find the right dose, and to watch for symptoms of intoxication. Mild dizziness, however, is common and will usually resolve without interruption of treatment.
  • Up to 12 sprays a day can be used, with a minimum gap of 15 minutes between each spray.
  • The patient should spray directly below the tongue or towards the inside of the cheek, varying the site at each dose. Avoid spraying on inflamed areas. The spray should never be aimed at the pharynx, as this can cause irritation.

Adverse effects[16]

Very common
Dizziness, fatigue.


  • Reduced or increased appetite.
  • Depression, disorientation, dissociation, euphoric mood.
  • Amnesia, balance disorder, disturbance in attention, dysarthria, dysgeusia, lethargy, memory impairment somnolence.
  • Blurred vision.
  • Vertigo.
  • Constipation, diarrhoea, dry mouth, glossodynia, mouth ulceration, nausea, oral discomfort, oral pain, vomiting.
  • Application site pain, asthenia, feeling abnormal, feeling drunk, malaise.
  • Falls.


  • Pharyngitis.
  • Hallucination (unspecified, auditory, visual), illusion, paranoia, suicidal ideation, delusional perception.
  • Syncope.
  • Palpitations, tachycardia.
  • Hypertension.
  • Throat irritation.
  • Abdominal pain (upper), oral mucosal discolouration, oral mucosal disorder, oral mucosal exfoliation, stomatitis, tooth discolouration.
  • Application site irritation.


No routine laboratory monitoring is required other than that necessary to monitor the patient's condition and any concomitant medication. It would seem prudent to monitor the patient to see whether continued administration of Sativex® is appropriate, although the manufacturers state that addictive potential is low.


  • Ketoconazole, ritonavir, rifampicin, carbamazepine, St John's wort and clarithromycin can potentiate the effects of Sativex®, so the dose may need to be re-titrated.
  • Sativex® should be used with caution in people taking digoxin.
  • Care should be taken with hypnotics, sedatives and drugs with potential sedating effects, as there may be an additive effect on sedation and muscle relaxation.
  • Patients already taking muscle relaxants may be at greater risk of falls.
  • Alcohol should be avoided, as co-ordination, concentration and reaction times may be affected.

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Further reading and references

  • Naftali T, Bar-Lev Schleider L, Dotan I, et al; Cannabis induces a clinical response in patients with Crohn's disease: a prospective placebo-controlled study. Clin Gastroenterol Hepatol. 2013 Oct11(10):1276-1280.e1. doi: 10.1016/j.cgh.2013.04.034. Epub 2013 May 4.

  • Moffat BM, Jenkins EK, Johnson JL; Weeding out the information: an ethnographic approach to exploring how young people make sense of the evidence on cannabis. Harm Reduct J. 2013 Nov 2710(1):34.

  • Schaub MP, Haug S, Wenger A, et al; Can reduce - the effects of chat-counseling and web-based self-help, Web-based self-help alone and a waiting list control program on cannabis use in problematic cannabis users: a randomized controlled trial. BMC Psychiatry. 2013 Nov 1413(1):305.

  • UK Cannabis Legal Guidelines

  1. Cannabis; DrugWise

  2. Drug Misuse: Findings from the 2015/16 Crime Survey for England and Wales

  3. Saugy M, Avois L, Saudan C, et al; Cannabis and sport. Br J Sports Med. 2006 Jul40 Suppl 1:i13-5.

  4. Sharma P, Murthy P, Bharath MM; Chemistry, metabolism, and toxicology of cannabis: clinical implications. Iran J Psychiatry. 2012 Fall7(4):149-56.

  5. Evins AE, Green AI, Kane JM, et al; Does using marijuana increase the risk for developing schizophrenia? J Clin Psychiatry. 2013 Apr74(4):e08. doi: 10.4088/JCP.12012tx2c.

  6. Rey, J; Does marijuana contribute to psychotic illness?, Current Psychiatry, Vol. 6, No. 2 / February 2007.

  7. Arseneault L, Cannon M, Poulton R, et al; Cannabis use in adolescence and risk for adult psychosis: longitudinal prospective study. BMJ. 2002 Nov 23325(7374):1212-3.

  8. Cannabis and mental health; The Royal College of Psychiatrists, February 2009

  9. Danovitch I, Gorelick DA; State of the art treatments for cannabis dependence. Psychiatr Clin North Am. 2012 Jun35(2):309-26. doi: 10.1016/j.psc.2012.03.003. Epub 2012 Apr 10.

  10. Lichtman AH, Martin BR; Cannabinoid tolerance and dependence. Handb Exp Pharmacol. 2005(168):691-717.

  11. Joshi M, Joshi A, Bartter T; Marijuana and lung diseases. Curr Opin Pulm Med. 2014 Mar20(2):173-9. doi: 10.1097/MCP.0000000000000026.

  12. Rabin RA, Zakzanis KK, Daskalakis ZJ, et al; Effects of cannabis use status on cognitive function, in males with schizophrenia. Psychiatry Res. 2013 Apr 30206(2-3):158-65. doi: 10.1016/j.psychres.2012.11.019. Epub 2012 Dec 13.

  13. Serpell MG, Notcutt W, Collin C; Sativex long-term use: an open-label trial in patients with spasticity due to multiple sclerosis. J Neurol. 2013 Jan260(1):285-95. doi: 10.1007/s00415-012-6634-z. Epub 2012 Aug 10.

  14. Public Information Report on Sativex® Oromucosal Spray; Medicines and Healthcare products Regulatory Agency (MHRA), December 2007 (archived content)

  15. Public Assessment Report, Decentralised Procedure, Sativex® Oromucosal Spray; Medicines and Healthcare products Regulatory Agency (MHRA)

  16. Summary of Product Characteristics (SPC) Sativex Oromucosal Spray®; GW Pharma Ltd, electronic Medicines Compendium, March 2015

  17. Perez J, Ribera MV; Managing neuropathic pain with Sativex: a review of its pros and cons. Expert Opin Pharmacother. 2008 May9(7):1189-95. doi: 10.1517/14656566.9.7.1189.

  18. Berman JS, Symonds C, Birch R; Efficacy of two cannabis based medicinal extracts for relief of central neuropathic pain from brachial plexus avulsion: results of a randomised controlled trial. Pain. 2004 Dec112(3):299-306.

  19. Kavia RB, De Ridder D, Constantinescu CS, et al; Randomized controlled trial of Sativex to treat detrusor overactivity in multiple sclerosis. Mult Scler. 2010 Nov16(11):1349-59. doi: 10.1177/1352458510378020. Epub 2010 Sep 9.

  20. Sibbald B; Conditional okay for cannabis prescription drug. CMAJ. 2005 Jun 21172(13):1672.

  21. Blake DR, Robson P, Ho M, et al; Preliminary assessment of the efficacy, tolerability and safety of a cannabis-based medicine (Sativex) in the treatment of pain caused by rheumatoid arthritis. Rheumatology (Oxford). 2006 Jan45(1):50-2. Epub 2005 Nov 9.

  22. Russo EB; Cannabinoids in the management of difficult to treat pain. Ther Clin Risk Manag. 2008 Feb4(1):245-59.

  23. Abrams DI; Integrating cannabis into clinical cancer care. Curr Oncol. 2016 Mar23(2):S8-S14. doi: 10.3747/co.23.3099. Epub 2016 Mar 16.