Overall, the prevalence of NMPAU () for prescription pain relievers in the past year was 1.8%, which translates into an estimated 3.7 million adults. The majority of these cases (80%, 2.96 million) did not meet the criteria for disordered use, defined by DSM-IV abuse and/or dependence. However, approximately 20% (733,000) of those who had used prescription analgesics non-medically in the past year did meet the criteria for disordered use, although the percentage of the total adult population is relatively small (<1%). Slightly less than 50% of all NMPAUs consumed on multiple occasions, such as monthly or more (1.7 million). Less than one-third of all NMPAUs (1.2 million) consumed only once or twice in the past year.
Population estimates for past year non-medical prescription analgesic use (NMPAU)
We examined the rates of NMPAU by different levels of pain (), and observed a positive association between pain and the rates of NMU (p<.05). Within the group exhibiting high pain, approximately 2.7% reported any NMPAU, compared with 2.4% in the moderate pain group and 1.3% in the no or minimal pain group. This represented a population estimate of 683,000, compared with 1.1 million NMPAUs with moderate pain and 1.8 million NMPAUs with no-or-minimal-pain. Similarly, disordered use was more common in the high-pain group (.75%) compared with the moderate (0.56%) and no-or-minimal-pain groups (0.21%).
reports the multivariate associations between pain, psychiatric and substance-use risk factors, and types of past-year NMPAU. More specifically, the strong and positive associations between physical pain and NMPAU observed in the bivariate cross-tables () remained significant after introduction of the substance use and psychiatric correlates. Compared with those classified as having no physical pain, the odds of non-disordered NMPAU in the past year were significantly higher for those with moderate (O.R.=1.66, 95% C.I.=1.22–2.25) and high (O.R.=2.18, 95% C.I.=1.56–3.03) levels of pain. This pattern was similar for disordered use, where compared with those with no physical pain, use was higher for those with moderate (O.R.=2.38, 95% C.I.=1.17–4.81) and high (O.R.=3.97, 95% C.I.=2.06–7.64) levels of physical pain. Overall, this suggests that physical pain exerts an independent influence, net of potentially confounding influences of demographic, psychiatric, and substance-use characteristics.
Multivariable correlates (odds ratios) of past year non-medical use (NMU) of prescription pain relievers
Substance use patterns also were significantly related to types of NMPAU, with prescription drugs (i.e., stimulants, benzodiazepines, tranquilizers) among the strongest predictors of both non-disordered and disordered NMPAU. Findings for the associations between psychiatric disorders and types of NMPAU, also shown in , reveal that mood disorders and anti-social personality disorder (ASPD) were not significantly related to non-disordered NMPAU (p>.05), but were positively and significantly associated (p<.05) with disordered NMPAU. In contrast, anxiety disorders were positively and significantly associated (p<.05) with non-disordered NMPAU but were not significantly related to disordered use. More specifically, anxiety disorders appeared to be more strongly associated with lower levels of NMPAU whereas mood disorders and ASPD were more strongly predictive of more harmful levels of use.
The results of the interaction tests between psychiatric and substance-use disorders by levels of pain are presented in . None of the interactions between psychiatric disorders and levels of pain were statistically significant for either type of NMPAU. Recall that the use of other prescription medications non-medically was the strongest predictor of both types of NMPAU. However, these effects did not vary by levels of pain, for either non-disordered (Wald-F=0.37, p=.689) or disordered (Wald-F=2.59, p=.200) levels of NMPAU.
Interactions between physical pain and selected risk factors on past year nonmedical use of prescription pain relievers
The findings for the interaction tests between illicit drug use and physical pain revealed only a marginally significant effect and only for disordered NMPAU (Wald-F=2.39, p=.061) but not for non-disordered use (Wald-F=1.73, p=.161). The probability of past-year disordered NMU was highest among those with high pain and disordered illicit drug use (pr=.04) compared with all other combinations of pain and types of illicit drug use. Additional inspection of the expected conditional values suggested that the probability of having high pain with no past-year illicit drug use was significantly higher (pr=.004) than for those with low pain and disordered use of illicit drugs (pr=.011).
The presence of an alcohol-use disorder significantly modified the risk of self-reported physical pain in the likelihood of both types of past-year NMPAU. To illustrate, the rates of NMPAU were significantly higher among those with an alcohol-use disorder compared with those without an alcohol-use disorder within each level of pain. The effect also appeared stronger for disordered NMPAU compared with non-disordered NMPAU, given that the magnitude of the differences in the predicted probabilities were substantially larger for disordered NMPAU.