Incidence trends of cannabis and cocaine use from periodic Spanish general population surveys: effect of standardizing results by age structure (2013)

Aims: This study estimates life-time incidence trends of cannabis and cocaine use over 38 years from general population surveys of drug use (GPSDU) in Spain, taking into account changes of population age structure.
Design: Periodic cross-sectional studies.
Setting: Eight biennial GPSDU from 1995 to 2009 in Spain.
Participants: Interviewees aged 15-64 years who reported age of first ever cannabis and/or cocaine use between 10 and 64 years between 1971 and 2008.
Measurements: Estimates of raw and standardized incidences were calculated as a weighted mean of the incidences from all surveys. Standardization was conducted to take into account changes of population age structure. Incidence trends were extracted applying weighted cubic smoothing splines to incidence estimates.
Findings: For both substances, estimated raw incidence trends increased up until 2000 (rates of 11.5 ± 0.7 and 3.6 ± 0.5 per 1000, respectively, for cannabis and cocaine), and then decreased significantly (in 2008, 9.6 ± 1.2 and 2.7 ± 0.6, respectively). In contrast, standardized rates exhibit a steadily increasing trend up to 2000 (9.0 ± 0.6 and 2.8 ± 0.4), followed by a statistically non-significant increasing trend afterwards (in 2008, 9.5 ± 1.2 and 2.8 ± 0.6). The largest increases of incidence were observed in both male and female subjects aged 15-19 years.
Conclusions: Using data from Spanish general population surveys of drug use, an apparently decreasing trend of raw incidence rates in both cannabis and cocaine use from 2000 became non-decreasing trends when these rates were standardized. First experiences of cannabis and cocaine use in Spain occur mainly in younger ages (15-19 years).
KEY FINDINGS:
An apparently decreasing trend in both cannabis and cocaine use in Spain from 2000 disappears when the rates are standardized to account for changes in the age structure of the population.