Heroin is an opioid drug made from morphine, which is a natural substance taken from the seed pod of various opium poppy plants grown in Southeast and Southwest Asia, Mexico, and Colombia. It can be a white or brown powder, or a black sticky substance known as black tar heroin. Heroin is highly addictive and can be injected, sniffed, snorted, or smoked. Some people mix heroin with crack cocaine, a practice called speedballing. The drug enters the brain rapidly and binds to opioid receptors on cells located in many areas, especially those involved in feelings of pain and pleasure and in controlling heart rate, sleeping, and breathing. The initial effects of heroin include a surge of euphoria or "rush," followed by a twilight state of sleep and wakefulness. Physical symptoms of use include drowsiness, respiratory depression, constricted pupils, nausea, a warm flushing of the skin, dry mouth, and heavy extremities. Heroin is highly addictive, and people who regularly use it often develop a tolerance, which means that they need higher and/or more frequent doses of the drug to get the desired effects. Continued use of the drug can cause issues, such as health problems and failure to meet responsibilities at work, school, or home, which is known as a substance use disorder (SUD) . Heroin is a Schedule I substance under the Controlled Substances Act, meaning that it has a high potential for abuse and no currently accepted medical use in treatment in the United States.