Progaba Gel 404b0c69Pharmacodynamics. Tramadol hydrochloride is a centrally acting synthetic opioid analgesic. Although its mode of action is not completely understood, from animal tests, at least two complementary mechanisms appear applicable: binding of parent and M1 metabolite to -opioid receptors and weak inhibition of reuptake of norepinephrine and serotonin. Tramadol, sold under the brand name Ultram...