How high is the risk of side effects of taking a multi-symptom medicine when you only have 1 symptom?

Question

ex. Tylenol Head Cold Medicine

Answer 1

Minimal negligible risk. Like Tylenol is for relief of fever and pain. If only headache is present, Tylenol does not reduce body temperature to subnormal. Most medicines don`t work when there is no need for them to work.