Exploring age of onset as a causal link between major depression and nonmedical use of prescription medications

BackgroundNonmedical use of prescription medications (NUPM) has been associated with major depression (MDD), but the specific processes by which they might interact and influence one another are understudied. This investigation attempted to clarify the relationship between MDD and NUPM by examining whether age of MDD onset influenced current and past NUPM and by examining whether age of NUPM onset influenced lifetime or past year MDD.MethodsThese goals were met through use of data from the 2005 to 2007 National Survey on Drug Use and Health. Analyses utilized design-based logistic regression, and current age and order of MDD onset and NUPM initiation were examined in interactions with age of MDD or NUPM onset.ResultsFor each year MDD onset was delayed, odds of lifetime, past year, past 30-day NUPM and substance dependence from NUPM were decreased by 2.3%, 2.6%, 1.9% and 2.3%, respectively. Earlier NUPM onset increased odds of past year (3.8%) and lifetime MDD (4.3%) in young adults, and lifetime MDD (2.5%) in 26–34 age group. Current age also interacted with age of MDD onset, with effects on NUPM pronounced in the 65 and older cohort. Order of MDD/NUPM onset generally did not interact with age of MDD onset, but it did interact with age of NUPM onset; the effects of NUPM onset on past year MDD were only significant in those with NUPM first.ConclusionsThese results highlight the need for further investigations of the interactions between depression and NUPM, particularly to evaluate potential causal relationships.