Heroin, also known as diacetylmorphine, is an opioid drug synthesized from morphine, which is found naturally in the opium poppy plant. Heroin usually appears as a white or brown powder.

When used in medicine, it is typically used to treat severe pain, such as that resulting from a heart attack or a severe injury. It is given via subcutaneous, intramuscular or intravenous route.

Illicit heroin is sometimes available in freebase form, dulling the sheen and consistency to a matte-white powder. Because of its lower boiling point, the freebase form of heroin is also often smoked and inhaled by snorting, in addition to being used intravenously.

Administration of heroin into the body, either through injection, inhalation or smoking delivers the drug to the brain very rapidly. Once in the brain, the heroin is converted back to morphine which binds to opiate receptors in areas of the brain, particularly those involved in the perception of pain and in reward.


When drugs such as heroin are used repeatedly over time, tolerance may develop. Tolerance occurs when the individual no longer responds to the drug in the way that they initially responded. In other words, it takes a higher dose of the drug to achieve the same level of response originally achieved.

Tolerance to drugs can be produced by several different mechanisms, and in the case of morphine or heroin, tolerance develops at the level of the cellular targets. After repeated activation of the opiate receptor by morphine, the enzyme adapts so that the morphine can no longer cause changes in cell firing. Thus, the effect of a given dose of morphine or heroin is diminished.

With repeated use of heroin, dependence also occurs. Dependence develops when the neurons adapt to the repeated drug exposure and only function normally in the presence of the drug. When the drug is withdrawn, several physiologic reactions occur. These reactions may range from mild to life threatening depending on the substance (e.g. coffee vs. heroin). This reaction is known as the withdrawal syndrome. The symptoms of withdrawal include restlessness, muscle and bone pain, insomnia, diarrhea and vomiting, among others. Users also experience severe craving for the drug during withdrawal, which can precipitate continued abuse or relapse.


Heroin abuse is associated with a number of serious health conditions, including fatal overdose, spontaneous abortion, and infectious diseases like hepatitis and HIV. Chronic heroin users may develop collapsed veins, infection of the heart lining and valves, abscesses, constipation and gastrointestinal cramping, and liver or kidney disease. Pulmonary complications, including various types of pneumonia, may result from the poor health of the user, as well as from heroin’s effects on breathing.

In addition to the effects of the drug itself, illicit forms of heroin often contains toxic contaminants or additives that can clog blood vessels leading to the lungs, liver, kidneys, or brain, causing permanent damage to vital organs.

Heroin abuse during pregnancy, together with related factors like poor nutrition and inadequate prenatal care, is also associated with low birth weight, a risk factor for later delays in the infant’s development. If the mother is regularly abusing the drug, the infant may be born physically dependent on heroin and could suffer from neonatal abstinence syndrome (NAS), a drug withdrawal syndrome in infants that requires hospitalization.