Outcomes reported in trials are usually for study participants at a high level of adherence to the study medicine and comparators. However, in usual practice lower levels of adherence are expected, potentially with different levels of adherence for different therapy options resulting from differences in side effects or methods of administration. In the absence of real-world evidence, an understanding of the relationship between effectiveness and adherence is required to project estimates of effectiveness in usual practice (with sub-optimal adherence) from efficacy reported in trials. However, the relationship (often non-linear) may be difficult to predict. Having real-world data on adherence alone is insufficient to estimate effectiveness.