What is white round pill hp 46?

The white round pill imprinted with "HP 46" is identified as Diphenhydramine Hydrochloride 25 mg, which is an over-the-counter antihistamine commonly used to relieve symptoms of allergy, hay fever, and the common cold. It can also be used as a nighttime sleep aid and to prevent motion sickness. Always consult a healthcare professional before using any medication to ensure it is appropriate for your needs.