As with any systematic review of observational studies, we may have missed studies meeting inclusion criteria if the AFD or outcome data were not prominently reported. We examined all studies included in a related systematic review,16 and went to great lengths in identifying further studies through subject experts. If there is significant publication bias and included studies represent a biased sample of all studies that have been undertaken, then overestimation of associations will have occurred. Given the somewhat contested nature of alcohol and drugs issues, this area seems highly vulnerable to publication bias.25 26 This possibility seems likely to occur where preliminary explorations of cohort study datasets are deemed to yield no interesting findings.16 It is striking that there were no reports drawn from birth cohort studies and we judge that the possibility of publication bias poses a profound threat to the certainty with which any inferences can be made on this subject.
