Introduction: The appropriate role of evidence in health policy decision making is controversial and requires more data on how decisions are actually made. Option B+ is a strategy to prevent mother to child transmission (PMTCT) of HIV that involves starting pregnant, HIV-positive women on triple drug antiretroviral therapy (ART) and continuing for life. It was rapidly adopted by sub-Saharan African countries with limited scientific evidence for its efficacy and safety, without waiting for the results from an ongoing randomised controlled trial (RCT) comparing PMTCT strategies. Methods: We interviewed 14 senior HIV policymakers in nine sub-Saharan African countries about factors influencing their adoption of Option B+. Results: While scientific evidence was important to the decision to adopt Option B+, policymakers were persuaded by data that did not come from RCTs. Other factors also played an important role including: evidence for ancillary benefits, simplicity, alignment with other values and priorities, and ease of integration with existing programmes. Conclusions: In adopting Option B+, gold-standard scientific evidence played a relatively minor role; other considerations were more important. Future research could help researchers determine whether these factors are influential in other contexts and to develop evidence that is more responsive to the needs of policymakers.