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Pharmacology: Pharmacodynamics: Pethidine is a narcotic analgesic that can be used for the relief of most types of moderate to severe pain. It interacts with stereospecific opiate receptors in the CNS and other tissues. Analgesia is produced primarily through an alteration in emotional response to pain. The relief of pain is fairly specific; other sensory modalities are essentially unaffected and mental processes are not impaired (unlike anaesthetics) except when given in large doses or to unusually susceptible individuals.
Pharmacokinetics: Absorbed by all routes of administration. After an IV dose of 0.8 mg/kg to normal subjects, plasma concentrations of about 0.2 μg/mL are obtained in 1 hour falling to 0.1 μg/mL after a further 2 hours; after an IM dose of 100 mg, plasma concentrations of about 0.5 μg/mL are attained in about 1 hour; the administration of anaesthetics appears to cause up to a two-fold increase in plasma concentration of unchanged drug; similarly plasma concentration appear to vary according to age, concentration being higher in older subjects.
Rapidly and extensively distributed throughout the tissues; volume of distribution, 200-300 liters; it readily crosses the placenta, is secreted in milk, and taken up to some extent by red blood cells; 40 to 65% bound to plasma proteins.
Metabolic reactions: N-demethylation, hydrolysis, Glucuronic Acid conjugation and N-oxidation.
Excretion: Influenced by pH value of urine. In acid urine, 10 to 30% of dose is excreted unchanged, 10 to 35% is excreted as Norpethidine, and about 1% is excreted as Pethidine N-oxide; in alkaline urine, less than 5% is excreted as Pethidine and Norpethidine. In normal subjects, about 70% of dose is excreted in the urine in 24 hours, up to 10% as unchanged drug, 10% as Norpethidine, 20% as Pethidinic Acid, 16% as conjugated Pethidinic Acid, 8% as Norpethidinic Acid, and 10% as conjugated Norpethidinic Acid.
