What is a side effect of giving someone prn medication?

PRN, when used in pharmacy or medicine, stands for pro re nata, meaning "as the circumstance arises". When you see PRN on a prescription, pharmacists usually translate this to as needed. For example, if you see a pharmacy "sig" which reads:

1 tab qs prn

a pharmacist would translate this to mean 1 tablet every night at bedtime as needed.

So there are no "side effects" of prn, other than maybe a decreased or increased tendency to take the medication, depending on the effects the medicine provides. For instance, if a medication causes nausea and vomiting, and PRN is included on the label, a patient might take the medication less often. On the other hand, if a medication causes euphoria, and includes PRN on the label, a patient might be inclined to take the medication more often.