Reliability of self-reported age of first drug use: analysis of New York City narcotics register data.

New York City Narcotics Register data were analyzed to determine the consistency of self-reported age of first drug use among multiple reports on a single individual. There were 15,263 individuals with two or more reports between January 1973 and April 1974 which specified age of first drug use. Discrepancies were noted in 68% of cases; in 31% the responses differed by 3 or more years. Even among reports from a single agency, there were discrepancies in 45% of cases. The degree of consistency was only slightly better when the analysis was restricted to subjects 20 years of age and younger. No comparable inconsistency was observed with respect to self-reported year of birth.