Every healthcare intervention comes with the risk, great or small, of harmful or adverse effects. A Cochrane review that considers only the favourable outcomes of the interventions that it examines, without also assessing the adverse effects, will lack balance and may make the intervention look more favourable than it should. This source of bias, like others, should be minimized. All reviews should try to include some consideration of the adverse aspects of the interventions.
This chapter addresses special issues relating to adverse effects in Cochrane reviews, with an emphasis on reviews in which adverse effects might be addressed using methods differing from those for other outcomes. Although in principle adverse effects are most reliably assessed using randomized trials, in practice many adverse events are too uncommon or too long term to be observed within randomized trials, or may not have been known when the trials were planned. A Cochrane review may use one of several strategies for addressing adverse effects, which differ in the extent to which the same methods are used to evaluate intended (beneficial) and unintended (beneficial or adverse) effects. The present chapter focuses on adverse effects that are usually taken to be unintended (Miettinen 1983). The different strategies for a review are discussed in Section 14.2.