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. Following oral, subcutaneous, or IM administration, onset of analgesia occurs within 10-15 minutes. Peak effects occur within 1 hour, declining gradually over 2-4 hours.
Meperidine is distributed widely, and it crosses the placenta and distributes into breast milk.
Accumulation after repeated or high doses, especially in patients with hepatic or renal impairment, should be considered. Acidification of the urine greatly enhances excretion of both meperidine and normeperidine. Due to the effects of opiate agonists on the gastrointestinal tract, meperidine should be used cautiously in patients with GI disease including GI obstruction, ulcerative colitis, or pre-existing constipation. Also, opiate agonists are contraindicated for use in patients with diarrhea secondary to poisoning or in patients with infectious diarrhea. Meperidine can slow the removal of the infecting organism or toxic material and should not be given until the condition has been treated.