A recent systematic review found that almost half of participants who take placebos in clinical trials experience drug related adverse events (AEs), with 5% of participants dropping out due to ‘drug related’ intolerance. However the placebo per se cannot be the cause of these adverse events. Instead, there are two overlapping likely explanations:

1. Misattribution
A patient may have an underlying condition whose natural history produces some event (such as a headache), then the patient misattributes the event to the placebo.

2.Nocebo effects
Having been warned about side effects in the patient information sheets, the patient may expect an adverse event. This negative expectation could then produce the event.

Nocebo effects may be caused—at least partly—by sharing information about AEs in the wrong way. This causes a tension between the ethical requirements of autonomy and non-maleficence. On the one hand, autonomy demands that patients be fully informed about treatment (adverse events). On the other hand, non-maleficence demands that patients be informed about AE’s in the right way. Ethical discussions of informed consent have focused almost exclusively on autonomy and may therefore been violating the requirement to do no harm. I will discuss ways in which autonomy and non-maleficence can be balanced in future clinical trials and ethical debates.