Pharmacokinetics

Oxytocin administered effectively by parenteral injection or nasal inhalation. Steady state, following parenteral administration, is usually achieved in plasma by 40 minutes.(3) Oxytocin’s plasma half-life is between 1 and 6 minutes. The drug distributes throughout the extracellular fluid, with minimal amounts reaching the fetus.

Oxytocinase, a glycoprotein aminopeptidase that is capable of degrading oxytocin, is produced during pregnancy and is present in the plasma. Enzyme activity increases gradually until term approaches, when there is a sharp rise in plasma levels and activity is high in the plasma, placenta and uterus. After delivery enzyme activity declines. Oxytocinase most likely originates from the placenta and regulates the amount of oxytocin in the uterus; there is little or no degradation of oxytocin in men, nonpregnant women, or cord blood. Oxytocin is rapidly removed from plasma by the liver and the kidneys, with only small amounts being excreted unchanged in the urine. Oxytocin is metabolized in the lactating mammary gland and is distributed into breast-milk.