A Systematic Review of Observational Studies on the Effectiveness of Opioid Therapy for Cancer Pain

Background: The prevalence of cancer-related pain and residual pain in cancer survivors is high. Opioids serve as the gold standard for treating moderate to severe cancer pain. The evaluation of the effectiveness of opioids in chronic non-cancer pain has shown a lack of effectiveness, or rather weak evidence for some of the drugs. In contrast, in cancer pain, opioids are expected to be very effective. Due to the nature of the disease, there is evidence of a paucity of randomized trials investigating opioid effectiveness in cancer pain on a long-term basis. Consequently, the effectiveness of opioids in managing cancer-related pain warrants further evidence-based review beyond randomized trials, including observational studies and case reports. Methods: The comprehensive literature search was conducted for the period 1996 through June 2010. Databases for the search included PubMed, EMBASE, Cochrane Reviews, and clinicaltrails.gov, along with reviews and cross references. Methodologic quality assessment of the observational studies managing chronic cancer pain with opioids was conducted utilizing the Agency for Healthcare Research and Quality (AHRQ) criteria for observational studies. Analysis of evidence included 5 levels of evidence developed by the United States Preventive Services Task Force (USPSTF) ranging from Level I to III with 3 subcategories in Level II. Grading recommendations were based on Guyatt et al's recommendations with 6 levels: 3 in the strong category and 3 in the weak category. Results: This evaluation is of 18 manuscripts considered for inclusion; 7 manuscripts met the inclusion criteria based on AHRQ quality assessment. Level of evidence for opioid therapy in cancer pain was Level II-3, and recommendations were 1C/strong recommendation based on observational studies, which could change based on future evidence. Conclusion: This systematic review of observational studies indicates Level II-3 evidence for effectiveness of opioids in cancer pain therapy, with 1C/strong recommendation based on observational studies, which could change based on future evidence.