What is a white oval pill with an s and 900 on it and the score in between?

The white oval pill marked with "S" and "900" on one side, with a score line in between, is identified as Diphenhydramine Hydrochloride 25 mg. Diphenhydramine is an antihistamine commonly used to relieve symptoms of allergy, hay fever, and the common cold, such as runny nose and sneezing. It is also used as a nighttime sleep aid and to prevent motion sickness. Always use medications as directed by a healthcare professional.