Side effects are ubiquitous in medicine and they often play a role in treatment decisions for patients and clinicians alike. Philosophers and health researchers often use side effects to illustrate issues with contemporary medical research and practice. However, technical definitions of ‘side effect’ differ among health authorities. Thus, determining the side effects of an intervention can differ depending on whose definition we assume. Here I review some of the common definitions of side effect and highlight their issues. In response, I offer an account of side effects as jointly (i) unintended and (ii) effects due to the causal capacities or invariances of an intervention. I discuss (i) by examining the intentions or reasons behind therapeutic interventions, and I discuss (ii) by appealing to a manipulationist model of causation. The analysis here highlights that side effects are conceptually distinct from related outcomes like adverse events, adverse drug reactions, and placebo effects. The analysis also allows for reflection on the utility of ‘side effect’ as a technical term in medical research and practice.

副作用在医学中普遍存在，并且常常在患者和临床医生的治疗决策中发挥作用。哲学家和健康研究人员经常使用副作用来说明当代医学研究和实践的问题。然而，卫生当局对“副作用”的技术定义有所不同。因此，根据我们假设的定义，确定干预措施的副作用可能会有所不同。在这里，我回顾了副作用的一些常见定义并强调了它们的问题。作为回应，我将副作用解释为（i）意外的和（ii）由于干预的因果能力或不变性而产生的影响。我通过检查治疗干预背后的意图或原因来讨论（i），并通过诉诸操纵主义因果关系模型来讨论（ii）。这里的分析强调，副作用在概念上与不良事件、药物不良反应和安慰剂效应等相关结果不同。该分析还可以反思“副作用”作为医学研究和实践中的技术术语的效用。