The Mail's story originally carried the headline: 'Just ONE cannabis joint 'can bring on schizophrenia' as well as damaging memory'. Although this headline was (slightly) modified later, Parker points out that the academic paper this article was 'based on' was:
not about cannabis, smoking or schizophrenia. Rather it is about an artificial compound that is not present in cannabis, which was injected into rats, and which led to changes in their brain waves.
The article was debunked at the time, including by Neurobonkers and Clear. In the latter, it says:
Dr Matt Jones, lead author of the study in question...told me that he was “disappointed but not surprised” at the Daily Mail’s coverage of his work. He gave me permission to quote him in saying that “This study does NOT say that one spliff will bring on schizophrenia”.
(Hat-tips to Craig Silverman and Kevin Arscott)