In the present study, the risk for potentially serious drug-drug interactions was increased in patients with MDD but to a lower degree than could be expected from their use of many drugs. One explanation for this may be that drug-drug interaction warnings based on the complete medication list of the patient are given in the MDD prescribing procedure. When prescribing to patients with OP, drug-drug interaction warnings only occur for drugs prescribed concomitantly, that is, the complete medication list is unavailable. Interestingly, previous results concerning MDD patients and Drug combinations that should be avoided are somewhat contradictory; the proportion of patients with such combinations was greater for patients with MDD than for patients with OP, but after adjustments for number of dispensed drugs, the odds including confidence interval was,1.0. The present study has several limitations. First, the crosssectional study design does not allow conclusions concerning causality between MDD and poor quality in drug treatment. Thus, we cannot rule out if MDD leads to low quality of drug treatment, or if low quality of drug treatment leads to MDD. Further longitudinal research is needed to clarify causality. Moreover, patients with and without MDD are obviously not alike. Other factors not included in the multivariate model may be of importance. However, we have tried to enhance precision and make the study more efficient by restricting the study population, that is, to only include older people with established obstructive pulmonary Veratramine disease, diabetes mellitus, and/or cardiovascular disease. Thus, the study population is a subset of all people $65 years in the Region Va��stra Go��taland. Furthermore, we have made an additional attempt to control for confounders by including important covariates in the model, such as burden of disease, psychiatric disease, and residence. Second, our analysis is based on register data only, and the estimated medication list may not Seratrodast reflect the true drug use, that is, drug use may be both over- and underestimated. Moreover, drugs are dispensed more frequently for patients with than without MDD. This may make the estimated medication list of a patient with MDD more accurate than that of a patient with OP. It cannot be ruled out that the differing registration frequency may have affected the results of the present study. However, the principle of estimating actual drug use based on prescriptions filled during a three months period has been employed in several previous studies, and indeed, the present method used for estimation of a medication list is the one used by the National Board of Health and Welfare for calculating quality indicators. Third, the drug-specific quality indicators employed in the present study do not provide all aspects of quality of drug treatment. Indeed, all the quality indicators in our study reflect inappropriate drug use.