Background Information: Drug & Alcohol Facts
There are many free sources of information about drugs and alcohol, some of them better than others.
Release provide regularly updated information about commonly used drugs and their effects, as does Frank (the latter being aimed at a generally younger audience). For a really comprehensive guide try DrugSearch (from DrugScope).
The health section of the BBC website includes some basic introductory information about alcohol - while the Institute of Alcohol Studies provides a range of free downloadable factsheets on alcohol and related issues.