This green, round pill imprinted with M 51 is a Amitriptyline Hydrochloride 25 MG Oral Tablet, per the NIH.
Amitriptyline Hydrochloride is a human prescription drug. The active pharmaceutical ingredient is amitriptyline hydrochloride and is classified as, "Tricyclic Antidepressant [EPC]."
This pill is made by a total of 3 firms: (1) Mylan Institutional Inc., (2) Mylan Pharmaceuticals Inc., and (3) Cardinal Health
There are 3 other green pills that with a similar imprint: "M 51". They may be a different dosage of the same drug or a different drug entirely.