High-potency cannabis and the risk of psychosis

The British Journal of Psychiatry (2009) 195: 488-491. doi: 10.1192/bjp.bp.109.064220
© 2009 The Royal College of Psychiatrists

Marta Di Forti, MD, MRCPsych, Craig Morgan, MSc, PhD, Paola Dazzan, MSc, PhD, MRCPsych, Carmine Pariante, MRCPsych, Valeria Mondelli, MD, PhD, Tiago Reis Marques, MD, Rowena Handley, BSc (Psychology), Sonija Luzi, BSc (Psychology), Manuela Russo, BSc (Psychology) and Alessandra Paparelli, MD –  Institute of Psychiatry, London
Alexander Butt, MD, MRCPsych
Springfield University Hospital, St George’s and South West Thames NHS Trust, London
Simona A. Stilo, MD, Ben Wiffen, BSc (Psychology), John Powell, MA, DPhil and Robin M. Murray, MD, DSc, FRCP, FRCPsych, FMedSci
Institute of Psychiatry, London, UK
Background
People who use cannabis have an increased risk of psychosis, an effect attributed to the active ingredient 9-tetrahydrocannabinol (9-THC). There has recently been concern over an increase in the concentration of 9-THC in the cannabis available in many countries.

Aims

To investigate whether people with a first episode of psychosis were particularly likely to use high-potency cannabis.

Method

We collected information on cannabis use from 280 cases presenting with a first episode of psychosis to the South London & Maudsley National Health Service (NHS) Foundation Trust, and from 174 healthy controls recruited from the local population.

Results

There was no significant difference between cases and controls in whether they had ever taken cannabis, or age at first use. However, those in the cases group were more likely to be current daily users (OR = 6.4) and to have smoked cannabis for more than 5 years (OR = 2.1). Among those who used cannabis, 78% of the cases group used high-potency cannabis (sinsemilla, ‘skunk’) compared with 37% of the control group (OR 6.8).

Conclusions

The finding that people with a first episode of psychosis had smoked higher-potency cannabis, for longer and with greater frequency, than a healthy control group is consistent with the hypothesis that 9-THC is the active ingredient increasing risk of psychosis. This has important public health implications, given the increased availability and use of high-potency cannabis.

Correspondence: Correspondence: Dr Marta Di Forti, Department of Psychiatry, Institute of Psychiatry, De Crespigny Park, London SE5 8AF, UK. Email: m.diforti@iop.kcl.ac.uk

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