Calculating additive treatment effects from multiple randomized trials provides useful estimates of combination therapies.

OBJECTIVE Many clinicians and decision makers want to know the combined effects of treatments that have not been evaluated in combination. It is possible to determine such treatment effects by making assumptions about the additive effects. We discuss here the prerequisites and methods of applying additivity assumptions in synthesizing the evidence from randomized trials and multiple treatment meta-analyses. STUDY DESIGN AND SETTING Using statistical approaches, we demonstrate the utility of additivity of both pairwise randomized trials and multiple treatment comparison meta-analyses. RESULTS We present illustratively an example on estimating the treatment effects of drug combinations for chronic obstructive pulmonary disease. We confirm the additive treatment effects by comparing with direct combination treatment trial results. CONCLUSION Additive effects may be a useful tool to estimate the effectiveness of treatment combinations.

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