Most of these involve the central nervous system and smooth muscle. The precise mechanism of action of hydrocodone and other opiates is not known, although it is believed to relate to the existence of opiate receptors in the central nervous system. In addition to analgesia, narcotics may produce drowsiness, changes in mood, and mental clouding. Radioimmunoassay techniques have recently been developed for the analysis of hydrocodone in human plasma. The analgesic action of acetaminophen involves peripheral and central influences, but the specific mechanism is as yet undetermined.
Antipyretic activity is mediated through hypothalamic heat regulating centers.
Acetaminophen inhibits prostaglandin synthetase. The behavior of the individual components is described below. The plasma half-life is 1.25 to 3 hours but may be increased by liver damage and following overdosage.