Taking someone else's prescription medication is not illegal, at least not in Connecticut. If you were to possess it and it was a narcotic it would be illegal, but if you were handed a pill by the person for whom the drug was prescribed and you took the pill immediately, you would not be violating any law of which I am aware.
However, it is still something that falls under the category of "drug use" within the realm of a law enforcement application. It should be disclosed, but if it was a one-time event I don't think it will cause a problem.
Cogito ergo summopere periculosus.
Fiat justitia, ruat coelum.