Smoking potent cannabis was linked to 24% of new psychosis cases analysed in a study by King’s College London.
The research suggests the risk of psychosis is three times higher for users of potent “skunk-like” cannabis than for non-users.
The study of 780 people was carried out by KCL’s Institute of Psychiatry, Psychology and Neuroscience.
A Home Office spokesman said the report underlines the reasons why cannabis is illegal.
Scientists found the risk of psychosis was five times higher for those who use it every day compared with non-users.
They also concluded the use of hash, a milder form of the drug, was not associated with increased risk of psychosis.
Psychosis refers to delusions or hallucinations that can be present in certain psychiatric conditions such as schizophrenia and bipolar disorder.