Undesirable effects can be an increase in an undesired outcome (e.g. more pain or more strokes), or a decrease in a desired outcome (e.g. worse vision or fewer healthy babies), in the treatment comparison group receiving the treatment of interest, compared to another comparison group. Undesirable effects can also include an increase in burden (e.g. more clinic visits or a more bothersome treatment).
If you feel that this definition hasn't helped you to understand the term, click on our monkey to let us know.