Methamphetamine use and psychotic symptoms: findings from a New Zealand longitudinal birth cohort.

Background: This study examined the association between methamphetamine use and psychotic symptoms in a New Zealand general population birth cohort ( n = 1265 at birth).
Methods: At age 18, 21, 25, 30, and 35, participants reported on their methamphetamine use and psychotic symptoms in the period since the previous interview. Generalized estimating equations modelled the association between methamphetamine use and psychotic symptoms (percentage reporting any symptom, and number of symptoms per participant). Confounding factors included childhood individual characteristics, family socioeconomic circumstances and family functioning. Long term effects of methamphetamine use on psychotic symptoms were assessed by comparing the incidence of psychotic symptoms at age 30-35 for those with and without a history of methamphetamine use prior to age 30.
Results: After adjusting for confounding factors and time-varying covariate factors including concurrent cannabis use, methamphetamine use was associated with a modest increase in psychosis risk over five waves of data (adjusted odds ratio (OR) 1.33, 95% confidence interval (CI) 1.03-1.72 for the percentage measure; and IRR 1.24, 95% CI 1.02-1.50 for the symptom count measure). The increased risk of psychotic symptoms was concentrated among participants who had used at least weekly at any point (adjusted OR 2.85, 95% CI 1.21-6.69). Use of methamphetamine less than weekly was not associated with increased psychosis risk. We found no evidence for a persistent vulnerability to psychosis in the absence of continuing methamphetamine use.
Conclusion: Methamphetamine use is associated with increased risk of psychotic symptoms in the general population. Increased risk is chiefly confined to people who ever used regularly (at least weekly), and recently.