Heroin affects nearly every major system in the body, from the brain’s reward circuitry to the heart, lungs, gut, immune system, and hormonal balance. Its effects begin within seconds of use and, with repeated exposure, create lasting changes that persist long after the drug wears off.
How Heroin Works in the Brain
Heroin activates opioid receptors, primarily the mu-opioid receptor, throughout the brain. This triggers a chain reaction: heroin suppresses the release of an inhibitory chemical (GABA) from certain brain cells, which in turn allows dopamine-producing neurons to fire freely. The result is a surge of dopamine in the brain’s reward system, producing intense euphoria, warmth, and pain relief. This same mechanism is what makes heroin so addictive, because the brain quickly learns to associate the drug with a powerful reward signal.
With chronic use, heroin physically damages the brain’s white matter, the wiring that connects different brain regions. This damage concentrates in pathways linking deeper brain structures to the prefrontal cortex, the area responsible for decision-making, impulse control, and weighing consequences. The result is measurable deficits in the ability to resist urges, evaluate risks, and regulate behavior. These cognitive impairments don’t just make daily life harder. They actively interfere with recovery by making it more difficult to stick with treatment and resist relapse.
Respiratory Depression and Overdose
The most immediately dangerous effect of heroin is respiratory depression. The drug slows breathing, makes it shallow, or can stop it entirely. When breathing slows enough, oxygen levels in the brain’s surrounding tissue drop rapidly. If oxygen delivery stops completely, unconsciousness occurs within about 17 to 20 seconds, and the brain begins depleting its fuel reserves within roughly three minutes. Without intervention, this sequence leads to brain cell death and can be fatal.
This is the mechanism behind nearly every heroin overdose death. The risk increases sharply when heroin is combined with other sedating substances like alcohol or benzodiazepines, or when a batch contains fentanyl, which is far more potent. Even outside of overdose, chronic heroin use depresses breathing enough to contribute to lung complications over time, including various types of pneumonia and a higher susceptibility to tuberculosis.
Effects on the Heart
People who inject heroin face a significant risk of infective endocarditis, an infection of the heart’s inner lining and valves. At one urban hospital studied between 1994 and 2000, 63% of all hospitalizations for this condition involved people who injected drugs. Among people who inject and present with a fever, about 13% show signs of heart valve infection on imaging, and that number jumps to 41% among those with bacteria in their bloodstream.
The infection most commonly strikes the tricuspid valve on the right side of the heart, and Staphylococcus aureus causes roughly 70% of cases. The consequences can cascade: infected clots break off and travel to the lungs, causing pulmonary abscesses, fluid buildup around the lungs, or blocked blood vessels. The valve itself can be destroyed, leading to severe leaking, right-sided heart failure, and abnormal heart rhythms including atrial fibrillation. Some of these complications, like ruptured infected aneurysms in the pulmonary arteries, can be fatal.
Gut and Digestive Problems
Heroin slows nearly every function in the digestive tract. It inhibits the muscular contractions that move food through the stomach and intestines, delays the release of digestive enzymes and bile, and increases fluid absorption from the intestines. The practical result is chronic, often severe constipation. Stool hardens, bowel movements become infrequent and painful, and many people experience straining and a persistent feeling of incomplete emptying.
Heroin also increases the tension in the anal sphincter, which further blocks normal defecation. These effects aren’t occasional side effects. They occur in the majority of regular opioid users and tend to persist as long as use continues, because the gut is densely packed with the same opioid receptors that heroin targets in the brain.
Immune System and Hormonal Disruption
Heroin suppresses the immune system through opioid receptors in the brain that regulate two key pathways: the stress hormone system (linking the brain to the adrenal glands) and the sympathetic nervous system (which controls the “fight or flight” response and communicates with immune organs like the spleen and lymph nodes). Activation of these pathways alters both innate immunity, your body’s first-line defenses, and adaptive immunity, the targeted response involving specialized immune cells.
Research on heroin self-administration shows significant reductions in T-cell function, the white blood cells critical for fighting infections and viruses. This pre-existing immune weakness likely contributes to the high rates of infectious diseases, including HIV and hepatitis, seen among people who use heroin. The vulnerability goes beyond shared needles; the immune suppression itself makes the body less capable of fighting off infections it encounters.
Heroin also disrupts the hormonal stress response. Studies have found that animals self-administering heroin showed stress hormone levels elevated by 79% to 198% compared to controls. In humans, chronic use disrupts the normal cycling of cortisol and other stress hormones, contributing to fatigue, mood instability, and difficulty managing stress without the drug.
Effects on Pregnancy and Newborns
Heroin crosses the placenta, and babies born to mothers who used heroin during pregnancy frequently develop neonatal abstinence syndrome, essentially going through withdrawal after birth. Symptoms typically appear within one to three days of delivery but can take up to a week, which is why affected newborns are monitored in the hospital for at least seven days.
The symptoms are wide-ranging and distressing: high-pitched or excessive crying, tremors, seizures, fever, rapid breathing, poor feeding, vomiting, diarrhea, increased muscle tone, and significant sleep disruption. Babies may also show excessive sucking, sneezing, sweating, and slow weight gain. The severity varies, and some infants require extended medical support to safely manage the withdrawal process.
What Withdrawal Feels Like
When a regular heroin user stops, withdrawal symptoms begin within 8 to 24 hours after the last dose and typically last 4 to 10 days. The experience includes nausea and vomiting, diarrhea, muscle cramps, hot and cold flushes, heavy sweating, watery eyes and runny nose, anxiety, and insomnia. While heroin withdrawal is rarely life-threatening in otherwise healthy adults, the intensity of symptoms is a major driver of continued use, as many people return to heroin simply to stop the discomfort.
The physical symptoms peak around days two through four and gradually subside, but sleep problems and anxiety can linger for weeks. The brain changes from chronic use, particularly the white matter damage affecting impulse control and reward processing, persist well beyond the acute withdrawal window and help explain why relapse rates remain high even after the physical symptoms resolve.