Heroin triggers a powerful surge of pleasure by flooding the brain with dopamine, then rapidly rewires the body to need more of it. Its effects range from an intense initial “rush” lasting seconds to minutes, to long-term damage to the brain, heart, lungs, and liver that builds over months and years of use. Here’s what happens at each stage.
What Happens in the Brain
Heroin travels through the bloodstream and crosses into the brain, where it converts into morphine and locks onto proteins called mu opioid receptors on the surface of brain cells. This triggers a reward circuit deep in the midbrain that releases a flood of dopamine, the chemical responsible for feelings of pleasure. The result is far more intense than what the brain produces naturally, which is a core reason heroin is so addictive.
Over time, the brain adapts. Receptor signaling becomes less sensitive, the receptors themselves decrease in number, and compensatory chemical processes ramp up to counterbalance the drug’s effects. This is tolerance: the same dose produces a weaker effect, pushing users toward higher and higher amounts. These cellular changes also reshape the brain’s stress and decision-making systems, making it progressively harder to feel normal without the drug.
The Immediate Effects
People who use heroin typically describe the initial experience as a surge of intense pleasure. This “rush” is usually accompanied by a warm flushing of the skin, dry mouth, and a heavy sensation in the arms and legs. Nausea and vomiting are common, especially early on. After the rush fades, users enter a drowsy, sedated state that can last several hours, during which thinking slows and heart rate drops.
The most dangerous immediate effect is respiratory depression. Heroin suppresses the brainstem’s drive to breathe, and at high enough doses, breathing can slow to a dangerous rate or stop entirely. When the brain doesn’t get enough oxygen, even briefly, it can suffer lasting damage. This is the primary mechanism behind fatal overdoses.
How the Body Changes With Repeated Use
Chronic heroin use damages nearly every major organ system. The lungs are particularly vulnerable. Heroin’s suppression of breathing, combined with the generally poor health that accompanies heavy use, increases the risk of pneumonia and tuberculosis. People who inject heroin face an additional layer of complications: scarred and collapsed veins, bacterial infections of blood vessels and heart valves (a condition called endocarditis), and abscesses at injection sites.
Brain imaging studies show deterioration of white matter, the tissue that connects different brain regions and allows them to communicate efficiently. This degradation affects the ability to make decisions, regulate behavior, and respond to stress. These changes can persist long after someone stops using.
Infectious Disease Risks
Sharing needles or other injection equipment carries a high risk of transmitting bloodborne infections. Hepatitis C is especially widespread among people who inject heroin. One large study of heroin-only users found that 66% tested positive for hepatitis C. HIV transmission is also a significant concern, though its prevalence varies by region and behavior patterns. People who use heroin alongside other injected drugs tend to have even higher rates of both infections.
Dependence and Withdrawal
Physical dependence develops quickly. As the brain’s chemistry adjusts to the constant presence of the drug, removing it triggers a predictable set of withdrawal symptoms. For heroin, these typically begin 8 to 24 hours after the last dose and last 4 to 10 days. Early symptoms include muscle aches, anxiety, sweating, and insomnia. These intensify into severe cramping, diarrhea, nausea, and vomiting as withdrawal peaks, usually around the second or third day.
Withdrawal is intensely uncomfortable but rarely life-threatening on its own. The greater danger is what comes after: because tolerance drops during a period of abstinence, people who relapse and take the dose they were previously accustomed to are at very high risk of overdose.
Overdose and Contamination
Heroin overdose deaths rose sharply in the 2010s, peaking at over 15,400 in 2016 before declining to roughly 3,984 in 2023. That decline doesn’t tell the whole story, though. By 2022, 80% of heroin-involved overdose deaths also involved illicitly manufactured fentanyl, a synthetic opioid that is far more potent. Much of what is sold as heroin now contains fentanyl or is entirely fentanyl, making every dose unpredictable. A quantity that would be survivable if it were pure heroin can be lethal if it contains even a small amount of fentanyl.
Overdose typically looks like extreme drowsiness progressing to unconsciousness, very slow or shallow breathing, bluish lips or fingertips, and pinpoint pupils. Naloxone, available as a nasal spray in most pharmacies, can temporarily reverse an opioid overdose by blocking the same receptors heroin activates.
Effects on Pregnancy
Heroin use during pregnancy can cause neonatal abstinence syndrome (NAS), a condition in which the newborn goes through withdrawal after delivery. For heroin specifically, symptoms tend to appear within the first 24 to 48 hours of life. Affected babies often show a characteristic pattern of increased muscle tone, tremors even at rest, and an exaggerated startle reflex.
Other symptoms include high-pitched excessive crying, irritability, sleep disturbances, and seizures. The gastrointestinal effects of heroin withdrawal are particularly severe in newborns, causing poor feeding, vomiting, and diarrhea that can lead to dehydration in infants whose metabolic systems are already fragile. Babies with NAS often require extended hospital stays and sometimes medication to safely manage the withdrawal process.