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adverse effects, undesirable treatment effects
Adverse events are unfavourable changes in health (or other outcomes), which may or may not be caused by treatments.
Patients’ and healthcare providers’ impressions and ideas about the effects of treatments are often the starting point for identifying harmful effects of treatments.
Commentary can go here if you want.
For example, participants may drop out of a study because of side effects of a treatment, and excluding these participants from the analysis could result in an overestimate of the effectiveness of the treatment, especially when the proportion dropping out varies by treatment comparison group.
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