Oxytocin is a drug of choice for stimulating contractions, which acts on the smooth muscle of the uterus and its responds, depends on the threshold of excitability of the uterine. The posterior pituitary along with ADH secretes oxytocin, which is similar to vasopressin that causes hypertension while synthetic oxytocin cause hypotension when given as a rapid IV bolus. Some studies suggest that the preservative chlorobutanol is responsible for these haemodynamic changes.