Objective: Surveys are considered the best source of data available on the prevalence of illicit drug use in the general adult population. The objective of the present study was to examine the consistency in self-reported lifetime use of illicit drugs. Method: Data were obtained from the Danish Health and Morbidity Surveys. A nationally representative subsample of the individuals invited in 2000 was also invited to the subsequent three survey waves (2005, 2010, and 2013). The baseline sample size included 4,803 individuals between ages 16 and 64 years, of whom 3,053 completed a self-administered questionnaire. Recanting was defined as reporting lifetime use at baseline but denying such use in at least one of the subsequent survey waves. Results: In all, 926 individuals reported lifetime use of cannabis in 2000 and 234 individuals reported lifetime use of hard drugs. After 5, 10, and 13 years, 10.2%, 15.6%, and 16.3% recanted their earlier reported use of cannabis, respectively. The prevalence increased with higher age. After 5 years, 20.4% recanted their earlier use of hard drugs. After 10 and 13 years, approximately 30% recanted their earlier use of hard drugs. This prevalence, too, increased with higher age. Conclusions: The validity of data on the lifetime use of illicit drugs derived from population surveys is questionable among adults in Denmark and perhaps in other countries. Future studies should take these findings into account when interpreting results, as such reporting patterns have considerable implications on the validity of the study results.