E for Ecstasy by Nicholas Saunders
Appendix 1: Reference Section

76 MDMA - Non-medical Use and Intoxication, by Ronald Siegel, from Journal of Psychoactive Drugs, Vol. 18/4 1986

This is a survey of a representative sample of drug users who had used MDMA at least twice in the previous year alongside other drugs. 44 such drug users answered a questionnaire. Siegel found that 90% of hard drug users who had tried MDMA did not want to repeat the experience - most found little or no effect and the rest did not enjoy it - and that samples contained about 20% less MDMA than was claimed by dealers, but none contained active impurities.