What is the right word for reaction but not exactly reaction [closed]

How would I describe that a person starts using a medical device and it had strange effects on him/her. It's kind of reaction but it's not exactly reaction because it is not all the time.

The device started or kicked off strange medical complication/behavior. What is the right word for that? Looking for formal word.

Thesaurus hasn't helped much.

Solution 1:

Well, you can use adverse effect.

Adverse Effect: An adverse effect is an undesired harmful effect resulting from a medication or other intervention, such as surgery. An adverse effect may be termed a "side effect", when judged to be secondary to a main or therapeutic effect. [Wikipedia]

Edit: Sorry just saw that @Weathervane has already written the same answer in the comment. But i think every answer in the comments are right.