Tehran University of Medical Sciences Basic & Clinical Cancer Research 2228-6527 16 3 2025 09 14 145 150 Vahideh Mohseni Cancer Research Center, Cancer Institute, Tehran University of Medical Sciences, Tehran, Iran Maryam Hadji 1- Cancer Research Center, Cancer Institute, Tehran University of Medical Sciences, Tehran, Iran Mina Khaki Cancer Research Center, Cancer Institute, Tehran University of Medical Sciences, Tehran, Iran Omid Nabavian Department of Anesthesiology, Imam Hospital Complex, School of Medicine, Tehran University of Medical Sciences, Tehran, Iran Kazem Zendehdel Cancer Research Center, Cancer Institute, Tehran University of Medical Sciences, Tehran, Iran Hamideh Rashidian Cancer Research Center, Cancer Institute, Tehran University of Medical Sciences, Tehran, Iran 2024 07 02 2025 06 06 Background: Underreporting bias related to opium use is a significant methodological issue that can threaten the results of epidemiologic studies, particularly when opium use is the exposure of interest. In our current study, we aimed to measure the sensitivity of opium use. among cancer patients and identify the contributing factors Method: In a cross-sectional study, we examined the sensitivity of self-reported opium use among cancer cases. In this study, we avoided using urine tests as the gold standard to prevent false positive results, given that most cancer patients use opioids to alleviate their pain. Instead, we relied on their reports of use to anesthesiologists as the gold standard and compared it with their reports to interviewers to calculate sensitivity Results: The sensitivity of self-reported opium use among cancer patients was approximately 63.33% (95% CI: 43.86% – 80.07%). Interestingly, this sensitivity was significantly higher among cigarette users 88.24% (95% CI: 63.56% – 98.54%) compared to non-users 30.77% (95% CI: 9.09% – 61.43%). Additionally, the sensitivity of self-reported opium use was higher among alcohol users and participants with low socioeconomic status compared to their counterparts, although these differences were not statistically significant. Conclusion: The observed sensitivity of self-reported opium use among cancer patients underscores the importance of meticulous and comprehensive approaches for collecting and interpreting self-reported substance use data. Researchers and policymakers should consider contributing factors to the sensitivity of self-reported opium use. https://bccr.tums.ac.ir/index.php/bccrj/article/view/530