Heroin produces a wide range of side effects that begin within seconds of use and, with repeated exposure, progressively damage nearly every organ system in the body. The immediate effects include slowed breathing, nausea, and extreme drowsiness. Over time, the drug reshapes brain chemistry, disrupts hormone production, and creates a physical dependence that causes painful withdrawal when use stops.
Immediate Physical Effects
Heroin binds to opioid receptors throughout the brain and body, triggering a cascade of physical changes within minutes. Users typically experience a warm flushing of the skin, dry mouth, and a heavy sensation in the arms and legs. Nausea, vomiting, and intense itching are also common during this initial “rush” phase.
After the rush fades, drowsiness sets in for several hours. Heart rate and breathing both slow significantly. Mental function becomes clouded, and users often drift in and out of consciousness in a state sometimes called “nodding.” Pupils constrict to tiny pinpoints, which is one of the most recognizable visible signs of opioid use.
Why Breathing Slows Down
The most dangerous immediate side effect is respiratory depression. Heroin activates opioid receptors on a small cluster of neurons in the brainstem that generate your breathing rhythm. Research published in eLife identified that as few as 70 to 140 neurons in this region are responsible for the drug’s ability to suppress breathing. When heroin silences these cells, the automatic drive to inhale weakens or stops entirely.
A second area in the brainstem also contributes to this effect, but the rhythm-generating cluster appears to be the primary site. This is why heroin overdoses kill: breathing slows to a dangerous rate or ceases altogether. The classic overdose presentation involves three simultaneous signs: pinpoint pupils, unconsciousness, and slowed or absent breathing.
Digestive and Gastrointestinal Problems
Constipation is the single most common gastrointestinal side effect of any opioid, and heroin is no exception. The drug activates opioid receptors lining the gut, which increases nonpropulsive muscle contractions in the intestines while also pulling more fluid out of stool. The result is hard, infrequent bowel movements that can become severely uncomfortable. Among people on chronic opioid therapy, an estimated 40% to 80% experience constipation, and some degree of it is nearly universal with regular use.
Beyond constipation, heroin commonly causes acid reflux, bloating, abdominal pain, and persistent nausea. These symptoms tend to worsen over time rather than improve, because the gut does not develop the same tolerance to opioids that the brain does.
Effects on the Brain Over Time
Chronic heroin use is linked to measurable changes in brain structure, including damage to white matter, the tissue that connects different brain regions and allows them to communicate efficiently. One recognized condition, toxic leukoencephalopathy, involves progressive destruction of this white matter and can cause symptoms ranging from confusion to severe disability.
Cognitive testing shows that long-term users tend to perform worse on tasks involving decision-making, impulse control, and mental flexibility. They have greater difficulty shifting strategies when circumstances change and show elevated compulsive traits. Interestingly, research has found that unaffected siblings of people with heroin dependence also show some of these same deficits, suggesting a combination of drug-induced damage and pre-existing vulnerabilities in how the brain handles impulse control.
Hormonal Disruption
Heroin interferes with the hormonal signals between the brain and the reproductive organs. In men, this often manifests as low testosterone, reduced sex drive, and erectile dysfunction. In women, it can disrupt or stop menstrual cycles entirely. Studies on active heroin users have documented abnormalities in luteinizing hormone, follicle-stimulating hormone, and prolactin, all of which play roles in sexual function and fertility. These changes can begin relatively early in the course of regular use and may not fully reverse after stopping.
Lung Damage and Infection Risk
People who inject heroin face a tenfold increased risk of community-acquired pneumonia compared with the general population. Several factors converge to make this happen: the drug itself suppresses the cough reflex, concurrent smoking damages the lungs’ natural defenses, and injection drug use weakens immune function over time.
Heroin can also cause a condition called noncardiogenic pulmonary edema, where fluid floods the lungs without any underlying heart problem. This can occur even with a single use and is a recognized cause of death in overdose situations. Other pulmonary complications include emphysema, blood clots that travel from injection sites to the lungs (septic embolization), and granulomas caused by injected filler materials lodging in lung tissue.
Infections From Injection
Injecting heroin introduces bacteria directly into the bloodstream, and the consequences can be severe. Skin and soft tissue infections, including abscesses and cellulitis, affect an estimated 17% to 37% of people who inject drugs. These infections account for nearly half of all hospitalizations in this population.
Injecting into muscle or under the skin rather than into a vein roughly quadruples the risk of abscesses. Black tar heroin, common in the western United States, is particularly damaging: users of this form are nearly eight times more likely to develop abscesses and lose significantly more usable veins over time. When bacteria from the skin enter the bloodstream, they can settle on heart valves and cause endocarditis, a life-threatening infection. They can also seed abscesses in the brain, spine, or internal organs.
Mental Health Effects
Depression and anxiety are strikingly common among people with opioid use disorder. A large meta-analysis covering more than 100,000 people found that 36% had current depression and 29% had current anxiety. Post-traumatic stress disorder affected about 18%, while ADHD was present in roughly 21%. These rates far exceed what you would find in the general population. Women with opioid use disorder were even more likely to experience depression, anxiety, and PTSD than men.
It can be difficult to untangle cause from effect. Some people use heroin to self-medicate pre-existing mental health conditions, but heroin also reshapes the brain’s reward and stress systems in ways that generate or deepen depression and anxiety. The cycle reinforces itself: worsening mental health drives continued use, and continued use worsens mental health.
Withdrawal and Physical Dependence
With regular use, the body adapts to heroin’s presence and begins to function abnormally without it. Physical withdrawal symptoms typically start 6 to 12 hours after the last dose and last approximately five days. Early symptoms include muscle aches, restlessness, sweating, runny nose, and anxiety. These intensify over the first two to three days into more severe symptoms: abdominal cramping, diarrhea, nausea, vomiting, dilated pupils, and goosebumps.
While heroin withdrawal is rarely fatal in otherwise healthy adults, it is intensely uncomfortable, and the fear of withdrawal is one of the strongest drivers of continued use. Sleep disturbances, irritability, and drug cravings can persist for weeks or months after the acute physical symptoms resolve, a phase sometimes called protracted withdrawal.
Overdose
Heroin overdose remains a leading cause of preventable death. The risk is highest when someone uses after a period of abstinence, because tolerance drops quickly but the amount used often does not. Mixing heroin with other sedating substances, particularly alcohol or benzodiazepines, dramatically increases the likelihood of fatal respiratory depression. The contamination of the heroin supply with fentanyl and its analogs has further escalated overdose rates, since these synthetic opioids are far more potent and can be present in amounts invisible to the user.