Major withdrawal symptoms peak between 48 and 72 hours after the last dose and typically subside after about a week; however, some individuals may show persistent withdrawal symptoms for months. Although heroin withdrawal is considered less dangerous than alcohol or barbiturate withdrawal, sudden withdrawal by heavily dependent users who are in poor health is occasionally fatal. Approximately 100 years after cocaine entered into use, a new variation of the substance emerged. This substance, crack, became enormously popular in the mid-1980s due in part to its almost immediate high and the fact that it is inexpensive to produce and buy. Cocaine is a strong central nervous system stimulant. Physical effects of cocaine use, including crack, include constricted blood vessels and increased temperature, heart rate, and blood pressure. Users may also experience feelings of restlessness, irritability, and anxiety Use of heroin in pregnancy also may increase the risk of a variety of birth defects. What is not entirely clear is whether these effects are directly due to the drug itself or related to the poor health behaviors that women who take heroin often have. It may also be caused by the substances that the heroin often is mixed with when it is made. | 
