It is difficult to evaluate all the published data concerning drugs of abuse in human pregnancy and breastfeeding. While some areas have been studied in depth (e.g. cocaine in pregnancy), most have not been investigated with any degree of thoroughness. Moreover, the data are frequently lacking in quality as well as quantity. Authors often fail to take account of confounding variables such as socioeconomic status or the use of other pharmacologically active substances when investigating the effects of one particular drug. Many of the older studies tend to omit vital details on dose of drug, frequency of use and trimester(s) of exposure. The data on drugs in breastfeeding are particularly poor, and there are similarly little data on long-term effects of in utero exposure. Where the literature approaches a consensus, the details available have been simply summarised without referral to individual papers. Where there are limited or conflicting data, the information available has been evaluated in the hope of offering the most useful advice.