Results
Young people aged 15-16 who smoked more than 10 cigarettes a day were three times more likely to have psychosis by the time they reached age 30
The earlier smoking was initiated, the greater the risk of developing psychotic symptoms, compared to those young people who began smoking later
Consistent with other studies and drug use generally, a dose-response relationship was found, so the more you smoke the greater the risk of psychosis.
Limitations
I am not convinced that the authors were able to adequately eliminate confounding factors such as cannabis use. Unfortunately, previous cannabis use by the cohort was assessed in a crude way: ‘Have you ever used marihuana or hashish (Yes/No)’. Given the association between cannabis and psychosis, asking one question about cannabis use is too simplistic. Those who responded ‘yes’ to this question could have used cannabis just once or multiple times. Equally there is no guarantee that those who replied ‘no’ have never used cannabis, as no independent check was made. The authors rely on self-reported measures which can be subject to recall bias.
The authors recognise that cannabis and tobacco are often used in combination and that they were unable to adequately account for this:
We cannot rule out the concurrent use of cannabis with tobacco smoking as an explanation for part or all of the associations reported in this study as cannabis users usually smoke cannabis mixed with tobacco and heavier smoking is associated with heavier cannabis use…