What is a oval white pill with 2 0 on it?

An oval white pill with "2 0" imprinted on it is identified as Diphenhydramine Hydrochloride 25 mg, which is an antihistamine commonly used to relieve symptoms of allergy, 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 before using any medication to ensure it is appropriate for your situation.