Substance misuse - The impact of using cannabis

Opinion is divided over whether this commonly used drug causes harm. Dr George Ryan explains.

Use is highest in ages 16-24
Use is highest in ages 16-24

Cannabis is a contentious substance, with conflicting opinions about its classification, its effects on mental health, and whether or not its use can lead to dependence.

In January 2009, the Home Office reclassified cannabis from Class C to Class B, which carries higher penalties for possession.

The main drivers for its reclassification were concerns about the effects of the drug (especially 'skunk', a strong type of cannabis) on the mental health of young people, and police concerns about increased seizures of skunk and an association with organised criminal gangs growing the drug on an industrial scale.

Prevalence
Cannabis is the most commonly used illicit drug in the UK, with 8.2 per cent of 16-59-year-olds in England and Wales using cannabis in 2006/7, and around 11 per cent of that age group using cannabis in Scotland.

Cannabis use is highest among people aged 16-24.

However, there has been a 20-25 per cent drop in the number of people of all ages using cannabis over the past five years.

Effects of cannabis
Cannabis is fat soluble, so its effects can continue for some time after its use has been discontinued. It can be detected in urine for up to a month and in oral fluid for up to 14 hours after its last use.

The Advisory Council on the Misuse of Drugs reported in November 2008 that in order to prevent one case of schizophrenia, it would be necessary for 5,000 young men or 20,000 young women to be prevented from ever having used cannabis.

These figures do not chime well with people who work in the field, who report anecdotally a much higher association between cannabis use and psychosis.

The evidence suggests that cannabis causes dependence with anxiety, restlessness, irritability, tiredness and insomnia occurring when a person ceases to use it.

However, the evidence does not consider cannabis to be a gateway drug leading to the use of opiates and cocaine. In fact, tobacco and alcohol are more likely to be gateway drugs than cannabis.

The effects of cannabis use in pregnancy are uncertain and there is little in the way of published evidence to suggest that there are significant long-term outcomes.

While a person is under its influence, cannabis leads to increased appetite (referred to among users as 'the munchies'), a relaxed feeling, euphoria, enhanced awareness and increased self-esteem. It also causes impaired co-ordination and increased reaction time.

It may cause anxiety, memory loss and paranoia - although again the evidence for this is equivocal.

The physical effects of cannabis use are lung damage associated with long-term use, vasoconstriction or vasodilatation (the latter causing the red eyes of a cannabis user), and a transient rise in pulse and BP.

Medical uses
With the exception of a cannabinoid-based spray, which is not licensed in the UK but may be provided on a named patient basis for sufferers of MS, it is illegal to produce or provide cannabis in any shape or form in the UK.

In contrast, cannabis is available in a number of US states to treat a variety of conditions, such as to increase appetite in cachectic patients - for example, patients with HIV/AIDS - or in HIV-associated neuropathy, bladder dysfunction and pain due to spasticity in MS.

There have also been trials of capsules for postoperative analgesia.

Conflicting experience
My clinical experience reflects the uncertainty alluded to above.

In general practice, I have seen two male patients suffer catastrophic effects on their mental health from their use of cannabis.

On the other hand, most patients I see when working in shared care or in a specialist service do not seem to suffer from any significant longor short-term adverse effects from their cannabis use.

In fact, many of them seem to derive some benefit from its use, especially with regard to anxiety and insomnia.

Treatment
The physical effects of cannabis use can be treated both by giving advice and on a symptomatic basis. The psychological effects present more of a challenge.

People who exhibit adverse psychological consequences from their cannabis use are more likely to present to mental health services than to addiction services - perhaps reflecting the paucity of treatment for problematic cannabis use.

General measures GPs can take to treat cannabis dependence and withdrawal include psychological support, advice about sleep hygiene and symptomatic relief of withdrawal symptoms, including nicotine replacement where appropriate.

  • Dr Ryan is a GPSI and shared care GP for Wolverhampton Addiction Services.

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