Produced by the Royal College of Physicians of Edinburgh and Royal College of Physicians and Surgeons of Glasgow


  • Dr SG Potts, Consultant Psychiatrist, Department of Psychological Medicine, Royal Infirmary of Edinburgh, Edinburgh, UK
  • Dr M Nolan, Specialist Registrar in Psychiatry, Department of Psychological Medicine, Royal Infirmary of Edinburgh, Edinburgh, UK


Cannabis is a psychoactive drug prepared from the plant Cannabis sativa. Its recreational use is widespread despite being illegal in most Western countries. Concern about the psychiatric effects of cannabis use outweighs concern for the drug’s physical toxicity, and largely underpins its continuing illegality, including the recent UK decision to reclassify it. In this article Drs Margot Nolan and Stephen Potts provide an informative overview of the main issues associated with cannabis use.

Key Points

  • Cannabis is a generic term for psychoactive drugs made in various ways from plants of the Cannabis genus, mainly Cannabis sativa.
  • Psychological dependence is common.
  • At usual levels of intake, cannabis produces a relaxed euphoria.
  • Taken in excess, it can produce a variety of unpleasant symptoms, ranging from anxiety and panic attacks to disorientation and disturbed behaviour.
  • Prolonged use is associated with an increased risk of developing schizophrenia and other psychoses.
  • The physical toxicity of cannabis is primarily the toxicity of the route of delivery, and since most cannabis is smoked, concern focuses on lung cancer, respiratory disease and cardiovascular problems.
  • Cannabinoids have a therapeutic role, which is currently limited and requires further research.
  • In the UK the legal status of cannabis was downgraded from class B to class C in 2004. This decision was reversed in May 2008.

Declaration of interests: No conflict of interests declared.

Historical background

Evidence of the use of cannabis in Asia, for medicinal purposes and in religious ritual, dates back several millennia and applies across various cultures, most notably the Hindus of the Indian subcontinent. Cultivation spread gradually from Asia to Africa, Europe and the Americas, and by the late nineteenth century its therapeutic use was well established in Western culture. Legal restrictions and taxation introduced in the early twentieth century, combined with the arrival of alternative treatments, pushed cannabis use into decline until the 1960s, when recreational use spread rapidly in the West, emerging from particular subcultures to embed within the wider population.

Forms, preparations, and terminology

Cannabis is a generic term for psychoactive drugs made in various ways from plants of the Cannabis genus, mainly Cannabis sativa. Marijuana is the herbal form, made from dried flowers and leaves, and usually consumed by rolling into a joint or reefer for smoking. Cannabis resin or hashish is a hard paste made from glandular trichomes found mainly on the flowers of female plants. It softens on heating, and is usually consumed by crumbling onto tobacco or into a water pipe to smoke. Cannabis can also be taken orally by baking it in cookies or other food, or infusing it as a tea. There is a particularly potent oil form (honey oil), made by extracting the active ingredients with a solvent, and this is usually inhaled after heating. There are many slang terms for cannabis, one of which, ganja, relates closely to ganjika, the ancient Sanskrit name for the herb.

Active ingredients

Extracts from the cannabis plant contain several hundred chemicals, many of which are psychoactive. The primary agent is delta-9-tetrahydrocannabinol, usually abbreviated to THC. Cannabis plants vary widely in their content of THC and other cannabinoids, with evidence that illicit breeding programmes have deliberately increased levels over the past 20 years from 1–3% to the 15–20% found in particularly potent forms such as ‘skunk’. This, coupled with the wide range of preparations and methods of consumption, means that the effects of cannabis, both sought and undesired, vary widely.

Mode of action

Delta-9-tetrahydrocannabinol and other components act on the endogenous cannabinoid system, first identified in the human brain in the 1980s. This system is believed to have a role in the regulation of many physiological functions, including control of movement, pain modulation and perception, coordination and balance, memory and learning, and pleasure sensations. To date, two receptor types have been identified. CB1 receptors are seen in high concentrations in the basal ganglia, hippocampus and cerebellum in the brain, and dorsal primary afferent pathways in the spinal cord, where they have an important role in pain perception. CB2 receptors are found in white blood cells, and are not involved in the psychoactive effects.

Psychological effects


At usual levels of intake, the effect produced (and sought) is a relaxed euphoria, so that users stereotypically sit around and chat companionably, often at length and with much laughter, before lapsing into sleep. Usual levels of cannabis can also alter visual perception, distort judgement of time and distance and induce carbohydrate craving.

Taken in excess in the acute phase, cannabis can produce anxiety and panic attacks, hallucinations (both visual and auditory), delusions (usually paranoid), disorientation and disturbed behaviour, although overdose is rarely directly fatal. Because the drug persists in the body, these symptoms can be prolonged for days, and occasionally weeks, after consumption of large amounts.


Most concern about cannabis relates to the psychiatric consequences of long-term use. The key features of physical dependence (tolerance and a withdrawal syndrome) do not often arise, but psychological dependence is common. Despite the impact of the 1936 film Reefer Madness, there is little to support a specific ‘cannabis psychosis’ linked to use of the drug. However, there is accumulating evidence that prolonged use is associated with an increased risk of developing schizophrenia and other psychoses, as well as an amotivational state and depression.

The risk of psychosis appears to greater when cannabis use is heavy, begins in early adolescence and occurs in those with a genetic vulnerability, as indicated by a family history. The risk is also greater with higher levels of THC, and in those with an established diagnosis of schizophrenia: continued cannabis use makes relapses more frequent, longer in duration and more difficult to treat, thereby worsening the outcome.

Physical effects

The toxicity of cannabis is primarily the toxicity of the route of delivery, and since most cannabis is smoked, concern focuses on lung cancer, respiratory disease and vascular diseases affecting the heart, brain and peripheries. There is conflicting evidence about the degree to which THC and other agents add to (or perhaps protect against) the risks of smoking itself. In utero exposure has been associated with low birth weight and height, neurological abnormalities, non-lymphocytic leukaemia, behavioural disturbance and learning problems. Salmonella, fungi and infectious bacteria have been cultured from marijuana, which may pose a risk for immunosuppressed patients using it therapeutically.

Medicinal use

The list of symptoms and conditions for which there are beneficial claims for cannabis is long and varied. In recent years, legal, prescribable preparations have been industrially developed for the pharmaceutical market in a number of forms, and with a currently restricted range of indications. The best evidence for efficacy relates to nabilone, a synthetic cannabinoid analogue of THC, prescribable in the UK for chemotherapy-induced nausea if other agents prove ineffective. Sativex, a buccal spray, is a cannabis extract containing THC and cannabidiol. It has been licensed in Canada for prescription use in neuropathic pain associated with multiple sclerosis since 2005, and for pain associated with cancer since 2007. Further clinical trials are under way, and may result in approval for prescription use in these conditions in the US, the UK and mainland Europe. Dronabinol (trade name Marinol) is a preparation of THC used in the US to treat wasting and nausea in HIV/AIDS and cancer, but it is not currently licensed in the UK.

Clinical trials are investigating the use of preparations of cannabis and cannabinoids in a number of neurological conditions, such as traumatic brain injury, and in some non-neurological conditions, such as irritable bowel disease and glaucoma. More research is required to identify indications, appropriate doses and adverse effects, before the medical use of these agents will be generally accepted in the UK.

Legal status

Concern about the psychiatric consequences of cannabis use, and fears cannabis might be a ‘gateway’ drug to other agents such as heroin and cocaine, underpin its continued illegality in most countries, although legalisation campaigns have been repeatedly mounted. In ‘coffee shops’ found – now in decreasing numbers – in parts of the Netherlands and some other Western countries, cannabis has achieved a semi-legal status, allowing consumers to purchase it without fear of arrest.

In 2004 the legal status of cannabis in the UK was downgraded from class B (alongside amphetamines) to class C (alongside GHB, ketamine and diazepam). This decision was reversed in May 2008, against the recommendations of the UK government’s own advisory body. The penalty for production and trafficking is the same for both drug classes, at 14 years, while the maximum sentence for possession of a class B drug is five years, against two years for class C. In fact, while it was placed in class C, possession of cannabis was much more likely to result in a warning or caution than prosecution and imprisonment, unless there were aggravating factors. In reclassifying the drug, the government has declared its wish for more robust enforcement of the law against supply and possession. In specific recognition of the psychiatric risks of cannabis use, new aggravating factors in sentencing will be introduced, including the supply of cannabis near mental health institutions.

Further reading

General information, with a focus on mental health

The Royal College of Psychiatrists. Cannabis and mental health. London: Royal College of Psychiatrists; 2006.

Report on reclassification

Advisory Council on the Misuse of Drugs. Cannabis: classification and public health. London: Home Office; 2008.

Potency and preparations

Hardwick S, King L. Home Office Cannabis Potency Study 2008. St Albans: Home Office Scientific Development Branch; 2008.

Toxicology and therapeutic use

The House of Lords. Science and Technology – Ninth Report: Cannabis: the scientific and medical evidence. London: House of Lords; 1998.