Meperidine alters the perception of pain in the spinal cord and CNS but has no effect on afferent nerve endings. Although the exact physiologic action of opiates is not known, stimulation of opiate receptors ultimately reduces neurotransmitter release. Respiration is depressed by a direct effect on the brain stem respiratory center, and the cough center is also suppressed by meperidine. Nausea and vomiting may be induced from direct stimulation of the chemoreceptor trigger zone. Opiate agonists increase smooth muscle tone, which can affect the intestinal, urinary, and biliary tracts.
Digestion is delayed by a decrease in gastrointestinal secretions.
Some references state that meperidine causes less biliary tract spasm than does morphine. Vasopressin output is stimulated, which can decrease urine output in conjunction with increased tone of the detrusor muscle. Miosis is another characteristic of the opioid analgesics. Compared with morphine, meperidine causes fewer of these effects. Meperidine, when administered orally, undergoes extensive first-pass metabolism.