A metabolic reduction injection was an injectable drug that would enable James T. Kirk and Ann Mulhall's Human bodies to host Sargon and Thalassa. Spock did not require one immediately to host Henoch, as his Vulcan body could accommodate a higher metabolism.
In 2268, Henoch (in Spock's body) volunteered to prepare the formula for the injection. Having done so, he enlisted Nurse Christine Chapel's aid in administering them to himself, and Thalassa (Mulhall), with a different formula for Sargon (Kirk). When Chapel objected, saying Kirk would die without the same formula, Henoch made her forget her objections. (TOS: "Return to Tomorrow")