Objective: Systematic-reviews of analgesics for low back pain generally include published data only. Obtaining data from unpublished trials is potentially important because they may impact effect sizes in meta-analyses. We determined whether including unpublished data from trial registries changes the effect sizes in meta-analyses of analgesics for low back pain.

Study Design and Setting: Trial registries were searched for unpublished data that conformed to the inclusion criteria of n=5 individual source systematic-reviews. We reproduced the meta-analyses using data available from the original reviews then re-ran the same analyses with the addition of new unpublished data.

Results: Sixteen completed, unpublished, trials were eligible for inclusion in four of the source reviews. Data were available for five trials. We updated the analyses for two of the source reviews. The addition of data from two trials reduced the effect size of muscle relaxants, compared to sham, for recent-onset low back pain from -21.71 (95%CI -28.23 to -15.19) to -2.34 (95%CI -3.34 to -1.34) on a 0-100 scale for pain intensity. The addition of data from three trials (one enriched design) reduced the effect size of opioid analgesics, compared to sham, for chronic low back pain from -10.10 (95%CI -12.81 to -7.39) to -9.31 (95%CI -11.51 to -7.11). The effect reduced in the subgroup of enriched design studies, from -12.40 (95%CI -16.90 to -7.91) to 11.34 (95%CI -15.36 to -7.32), and in the subgroup of non-enriched design studies; from -7.27 (95%CI -9.97 to -4.57) to -7.19 (95%CI -9.24 to -5.14).

Conclusion: Systematic-reviews should include reports of unpublished trials. The result for muscle relaxants conflicts with the conclusion of the published review and recent international guidelines. Adding unpublished data strengthens the evidence that opioid analgesics have small effects on persistent low back pain and more clearly suggests these effects may not be clinically meaningful.


back pain, analgesics, publication bias, clinical trials (as topic), meta-analysis (as topic)

Link to Publisher Version (URL)


Find in your library