A narcotic is an addictive drug that reduces pain, induces sleep and may alter mood or behaviour. In US legal context, narcotic refers to opium, opium derivatives, and their semi-synthetic or fully synthetic substitutes as well as cocaine and coca leaves, which although classified as "narcotics" in the U.S. Controlled Substances Act (CSA), are chemically not narcotics. Drug effects depend heavily on the dose, route of administration, previous exposure to the drug, and the expectation of the user. Aside from their clinical use in the treatment of pain, coughing and acute diarrhea, narcotics produce a general sense of well-being known as euphoria by reducing tension, anxiety, and aggression. Narcotic use is associated with a variety of effects including drowsiness, itching, sleeplessness, inability to concentrate, apathy, lessened physical activity, constriction of the pupils, dilation of the subcutaneous blood vessels causing flushing of the face and neck, constipation, nausea, vomiting and, most significantly, respiratory depression. Among the hazards of careless or excessive drug use are the increasing risk of infection, disease and overdose.