Background. Pairwise and network meta-analyses using fixed effect and random effects models are commonly applied to synthesize evidence from randomized controlled trials. The models differ in their assumptions and the interpretation of the results. The model choice depends on the objective of the analysis and knowledge of the included studies. Fixed effect models are often used because there are too few studies with which to estimate the between-study SD from the data alone.Objectives. The aim of this study was to propose a framework for eliciting an informative prior distribution for the between-study SD in a Bayesian random effects meta-analysis model to genuinely represent heterogeneity when data are sparse. Methods. We developed an elicitation method using external information, such as empirical evidence and expert beliefs, on the ‘range’ of treatment effects to infer the prior distribution for the between-study SD. We also developed the method to be implemented in R. Results. The 3-stage elicitation approach allows uncertainty to be represented by a genuine prior distribution to avoid making misleading inferences. It is flexible to what judgments an expert can provide and is applicable to all types of outcome measures for which a treatment effect can be constructed on an additive scale. Conclusions. The choice between using a fixed effect or random effects meta-analysis model depends on the inferences required and not on the number of available studies. Our elicitation framework captures external evidence about heterogeneity and overcomes the assumption that studies are estimating the same treatment effect, thereby improving the quality of inferences in decision making.