Ultram acts on the central nervous system and unlike codeine, it is a totally synthetic analgesic compound. Although its mode of action is not completely understood, from animal tests, at least two complementary mechanisms appear applicable: the binding of parent compound to opioid receptors and weak inhibition of reuptake of norepinephrine and serotonin by nerves located within the central nervous system. These mechanisms may contribute independently to the overall analgesic profile of Ultram. Analgesia in humans begins approximately within one hour after administration and reaches a peak in approximately two to three hours.