An aftereffect is exactly what it sounds like it is. It is an effect that occurs after the primary effect of an action has already run its course.
When talking about medical treatments, reported aftereffects are usually negative because most people are natural born complainers so they don’t feel it’s worth talking about positive aftereffects. However, there is nothing in the word itself that requires that an aftereffect has only negative consequences.
If you think it sounds like I’m complaining about most people being natural born complainers, you are right. I am complaining about that.
Why am I complaining about people being natural born complainers? I would have thought the answer was obvious. I’m a natural born complainer.