What is white round pill with P on one side and z or n on the other?

A white round pill with a "P" on one side and either a "Z" or "N" on the other is typically identified as Diphenhydramine Hydrochloride 25 mg, an over-the-counter antihistamine used to relieve symptoms of Allergies, hay fever, and the common cold. It can also be used as a sleep aid due to its sedative properties. Always consult a healthcare professional for accurate identification and advice regarding medications.