Oxytocin is a hormone produced by the hypothalamus but which is stored and secreted by the pituitary. It works in a manner similar to a neurotransmitter, modulating certain other chemical reactions in the brain. It is produced in particularly large quantities during labor, and appears to play a role in orgasm, intimacy and anxiety. In addition, problems with oxytocin receptors are correlated with aggressive behavior.
In it's pharmaceutical preparation, it is used to induce and support labor and to treat postpartum bleeding. As it is destroyed by the digestive tract, it must be administered by an alternative method, generally injection or with a nasal spray. It rarely has side effects in standard doses, and what side effects it has tend to be mild. However, it can be dangerous if used in large doses or over a period of more than 24 hours as it can cause extreme contractions of the uterus, rupture of the uterus and can actually induce postpartum bleeding. Oxytocin at Wikipedia