Meta-analyses to evaluate analgesic interventions: a systematic qualitative review of their methodology.

A systematic search of the literature was performed to identify the maximum possible number of meta-analyses that evaluated analgesic interventions. Seventy-four reports were identified and retrieved and the scientific quality of 80 separate meta-analyses was assessed under blind conditions by 2 judges using Oxman and Guyatt's index. Most of the meta-analyses evaluated pharmacological interventions for chronic pain conditions and two-thirds were published since 1990. Ninety percent of the meta-analyses had methodological flaws that could limit their validity. The main deficiencies were lack of information on methods to retrieve and to assess the validity of primary studies and lack of data on the design of the primary studies. Meta-analyses of low quality produced significantly more positive conclusions. For several topics, different meta-analyses evaluating the same intervention produced conflicting results. The need to resolve these contradictions is highlighted.