What is 503 3mg green pill?

The 503 green pill imprinted with "503" is identified as Diphenhydramine Hydrochloride 25 mg, an over-the-counter antihistamine. It's commonly used to relieve symptoms of allergy, hay fever, and the common cold, such as runny nose, sneezing, and itching. Additionally, it's often used as a sleep aid due to its sedative effects. Always use medications as directed and consult a healthcare professional if you have questions or concerns.