Synacthen Depot is a long-acting injectable medication primarily used in diagnostic testing and treatment of certain medical conditions. It contains tetracosactide, a synthetic form of adrenocorticotropic hormone (ACTH), which stimulates the adrenal glands to produce cortisol and other corticosteroids. This action makes it useful in diagnosing adrenal insufficiency and differentiating between primary and secondary adrenal disorders. In therapeutic settings, Synacthen Depot is employed in the management of conditions such as multiple sclerosis, rheumatoid arthritis, and certain types of skin conditions, leveraging its ability to modulate immune response and inflammation. The depot formulation provides a sustained release of the active ingredient, allowing for less frequent dosing compared to immediate-release forms.