Andrew Johns, MB, Consultant in Forensic Psychiatry at the Maudsley Hospital in London, noted in a Feb. 2001 article, "Psychiatric Effects of Cannabis," published in the British Journal of Psychiatry:

“There is good evidence that taking cannabis leads to acute adverse mental effects in a high proportion of regular users. Many of these effects are dose-related, but adverse symptoms may be aggravated by constitutional factors including youthfulness, personality attributes and vulnerability to serious mental illness…

An appreciable proportion of cannabis users report short-lived adverse effects, including psychotic states following heavy consumption, and regular users are at risk of dependence…

The untoward mental effects of cannabis may be classified:

  1. Psychological responses such as panic, anxiety, depression or psychosis. These effects may be described as ‘toxic’ in that they generally relate to excess consumption of the drug.
  2. Effects of cannabis on pre-existing mental illness and cannabis as a risk-factor for mental illness.
  3. Dependency or withdrawal effects.”
Feb. 2001