1 Matthew L The need for health warnings about cannabis and psychosis. We need to be cautious when calling for health warnings as Mathew Large does on the issue of cannabis and psychosis.Large asserts that “almost nothing scientific was known about the side effects of cannabis when cannabis use became popular in the second half of the past century” and goes on to compare this absence of knowledge with contemporary teenagers’ access to distorted information via the internet about the benefits of cannabis. I would suggest that we need to be careful in the assumptions that we make about how far scientific knowledge has advanced in exploring the links between cannabis and mental health.

2 Addington J

Addington D Patterns, predictors and impact of substance use in early psychosis: a longitudinal study. , 3 Moore TH

Zammit S

Lingford-Hughes A

et al. Cannabis use and risk of psychotic or affective mental health outcomes: a systematic review. 4 ElSohly MA

Mehmedic Z

Foster S

Gon C

Chandra S

Church JC Changes in cannabis potency over the last two decades (1995-2014): analysis of current data in the United States. 5 Hamilton I

Galdas PM

Essex HN Cannabis psychosis, gender matters. Many of the seminal and frequently cited epidemiological studies and systematic reviews on this issue are outdated and largely irrelevant (seefor examples) because the type of cannabis that teenagers and others are supplied has changed substantially since the second half of the last century. Several proxy measures of cannabis availability have identified a trend of increasing tetrahydrocannabinol and decreasing cannabinoid concentrations in cannabis.Exposure to these high-potency forms of cannabis and their effect on mental health is still not fully understood. Beyond the limitations already mentioned in how scientific knowledge has been gathered so far, a consistent over-reliance on self-reported data, an absence of attention to confounding variables such as polysubstance use, and crude binary questions or sampling of cannabis and non-cannabis use have occurred. Even basic demographic information such as sex requires more attention than at present.

Referral to cannabis as though it were one type of drug is unhelpful and misleading; researchers, clinicians, and policy makers need to start differentiating the various forms that cannabis takes in their various endeavours. I agree with Large that “young people deserve the best medical information”, but we are not yet in a position to provide the best medical information or health warnings about cannabis and psychosis until we put our own house in order and address some of these basic problems. If we do provide this information, it needs to be accurate, credible, and, most importantly, heard.

I declare no competing interests.

Article Info Publication History Identification DOI: https://doi.org/10.1016/S2215-0366(16)00086-9 Copyright © 2016 Elsevier Ltd. All rights reserved. ScienceDirect Access this article on ScienceDirect