Background: Despite hints about the high incidence of pain patients misreporting their pain medication use, there are only a few non-controlled studies on the topic that focus solely on opioids. Objective: Using toxicological analyses in a cross-sectional study, we investigated patients’ reliability regarding their report of any current pain medication use. Study Design: A cross-sectional study. Setting: A comprehensive pain center and a surgical unit of a German University Hospital. Methods: Consecutive outpatients at their first visit to the pain clinic (PG, n = 243) and pre-surgical control patients (SG, n = 100) suffering from pain reported on their current pain medication. The patients’ reports were verified in serum and urine using specific toxicological methods. Two types of noncompliance were defined: under-reporting (detection of non-reported substances) and overreporting (reported substances undetectable). The impact of clinical parameters on compliance was investigated using binary logistic regression. Results: The incidence of noncompliance was significantly higher in the PG (43.3%) than in the SG (24%; P < 0.05). Under-reporting occurred similarly in both groups (31% PG; 23% SG), whereas over-reporting predominantly appeared in the PG (11% vs. 2%; P < 0.05). Opioids were not most frequently under-reported, but the highest proportion of under-reported drugs (underreported in relation to detection incidence) was found for non-opioid analgesics (NSAIDs: 29% PG; 25% SG; other: 42% PG; 32% SG) and psychotropic drugs (35% PG; 53% SG). In the PG, logistic regression revealed high depression scores to be predictive for noncompliance (odds ratio 2.12). Limitations: Due to lack of a structured follow-up interview motives of under- and over-reporting stay speculative. Conclusions: Under-reporting of non-opioid analgesics is the main type of noncompliance, a disquieting fact in light of their toxicity and adverse effects. Further research is required in terms of drug assessment and compliance improvement strategies in pain clinics; therefore, toxicological monitoring is indispensable. Clinical Trial: NCT01625065; Medi-3889-10 Key words: Medication compliance, adherence, chronic pain, toxicological analyses, urine drug testing, NSAID, opioids

This study is based on face-to-face interviews with patients, including details about side effects and fracture history, providing a more pronounced picture of the relation of opioid intake and risk of fracture.