Your grandmother was right after all. Yes, cannabis is bad you for - in case you haven't found out yet.

In fact, cannabis increases the risk of psychotic illness later in life by 40% - according to a Cardiff University collaborative study.

And I thought in this age of crack cocaine nobody takes a second look at cannabis (or marijuana), much less use it.

However, it apparently remains the most commonly used illegal substance in most countries including the UK and USA.

The study, funded by the Department of Health, found that individuals who had used cannabis were 41 per cent more likely than those who had never used the drug to have any psychosis (presence of delusions or hallucinations).

The risk increased relative to dose, with the most frequent cannabis users more than twice as likely to have a psychotic outcome.

Depression, suicidal thoughts, and anxiety outcomes were examined separately, and findings for these outcomes were less consistent, with fewer attempts made to address non-causal explanations than for psychosis.

Find more details from the full report.

[Photo Credit: Cardiff University]