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Pharmacology: Pharmacodynamics: Morphine acts as agonists interacting with stereospecific and saturable binding sites or receptors in the brain and other tissues. They are present in highest concentration in the limbic system, thalamus, hypothalamus, midbrain and spinal cord. The affinity of opioid analgesics for the binding sites correlates well with their potency as analgesics.
Pharmacokinetics: Absorption: Rapidly absorbed after subcutaneous or intramuscular administration with rapid disappearance from plasma, especially after IV administration.
Blood Concentration: After IV injection of 10 mg, serum concentrations of about 60 ng/mL are obtained in 15 minutes falling to 30 ng/mL after 30 minutes and to 10 ng/mL after 3 hours. After an intramuscular dose of 10 mg, peak serum concentrations of 70 to 80 ng/mL are attained in 10 to 20 minutes; subcutaneous doses given similar concentrations to intramuscular doses at 15 minutes but remain slightly higher during the following 3 hours.
Excretion: After a parenteral dose, about 90% is excreted in 24 hours, with about 10% as free morphine, 65 to 75% as conjugated morphine, 1% as normorphine and 3% as normorphine glucuronide.
Distribution: Widely distributed throughout the body, mainly in the kidneys, liver, lungs and spleen; lower concentrations appear in the brain and muscles; morphine crosses the placenta and traces are secreted in sweat and milk; protein binding, about 35% bound to albumin and to immunoglobulins at concentrations within the therapeutic range.
Metabolic Reactions: Inactivated in the liver, primarily by conjugation with glucuronic acid.
Half-Life: Serum half-life in the period 10 minutes to 6 hours following intravenous administration, 2 to 3 hours; serum half-life in the period 6 hours onwards, 10 to 44 hours.
