Neonatal withdrawal or neonatal abstinence syndrome (NAS) is a withdrawal syndrome of infants, caused by the cessation of the administration of licit or illicit drugs. Tolerance, dependence, and withdrawal may occur as a result of repeated administration of drugs or even after short-term high dose use—for example during mechanical ventilation in intensive care units. There are two types of NAS: prenatal and postnatal. Prenatal NAS is caused by discontinuation of drugs taken by the pregnant mother, while postnatal NAS is caused by discontinuation of drugs directly to the infant.