They slow down various parts of the body and affect the central nervous system. Moderate doses cause euphoria, drowsiness, lethargy, respiratory depression, constricted pupils, constipation, and nausea. Overdoses can cause slow, shallow or absent breathing, clammy skin, convulsions, coma, and possible death. Sudden withdrawal results in watery eyes, runny nose, restlessness, rapid pulse, elevated blood pressure, diarrhea, loss of appetite, irritability, tremors, panic, chills and sweating, cramps and nausea. Moderate doses can result in motor disturbances, anxiety, paranoia, delusions of persecution, illusions and hallucinations, and poor perception of time and distances.
Flashbacks can occur months or years after the original dose.