“Use of illicit drugs, particularly cannabis, by young people is widespread and is associated with several types of psychological and social harm. These relations might not be causal. Causal relations would suggest that recreational drug use is a substantial public health problem…

All these associations seemed to be explicable in terms of non-causal mechanisms.

Available evidence does not strongly support an important causal relation between cannabis use by young people and psychosocial harm…

The lack of evidence of robust causal relations prevents the attribution of public health detriments to illicit drug use.”

http://www.ncbi.nlm.nih.gov/pubmed/15145631