Mechanism of action
Heroin (diacetylmorphine) is a prodrug that rapidly crosses the blood-brain barrier due to high lipid solubility conferred by acetyl groups at the 3- and 6-positions of the morphine molecule. Once in the central nervous system, heroin is rapidly deacetylated by esterases to 6-monoacetylmorphine (6-MAM) and then to morphine, both of which are active metabolites.
The analgesic and euphoric effects result from potent mu-opioid receptor (MOR) agonism in the brain and spinal cord. Activation of MOR in the ventral tegmental area and nucleus accumbens triggers dopamine release in reward pathways, producing intense euphoria and reinforcing addictive behavior. In the brainstem respiratory centers, MOR activation decreases responsiveness to carbon dioxide, leading to respiratory depression—the primary cause of fatal overdose.
Heroin also has activity at delta- and kappa-opioid receptors, though mu-receptor effects predominate. Chronic use leads to receptor desensitization, downregulation, and neuroadaptive changes that underlie tolerance, physical dependence, and the withdrawal syndrome upon cessation.
- Heroin overdose triad: respiratory depression, CNS depression (coma), and miosis (pinpoint pupils).
- Naloxone is the antidote for opioid overdose but has a shorter half-life than heroin, requiring monitoring for re-sedation.
- 6-Monoacetylmorphine (6-MAM) in urine is specific for heroin use; morphine alone can be from prescription opioids.
- Heroin withdrawal is very uncomfortable but rarely life-threatening, unlike alcohol or benzodiazepine withdrawal.
- IV heroin use is a leading cause of right-sided (tricuspid) endocarditis, most commonly from Staphylococcus aureus.
- Fentanyl contamination of street heroin has dramatically increased overdose mortality due to unpredictable potency.
- Medication-assisted treatment with methadone or buprenorphine is the gold standard for opioid use disorder and reduces mortality.
- Pregnant women using heroin should be transitioned to methadone or buprenorphine to reduce neonatal abstinence syndrome and improve outcomes.
Indications
In the United States, heroin has no approved medical indications and is classified as a Schedule I controlled substance, indicating high abuse potential and no accepted medical use.
In the United Kingdom and some other countries, pharmaceutical diamorphine (heroin) is used for:
- Severe acute pain (myocardial infarction, trauma, postoperative)
- Cancer-related pain unresponsive to other opioids
- Palliative care
In clinical practice in the US, heroin is encountered exclusively in the context of:
- Opioid use disorder requiring detoxification and treatment
- Acute opioid toxicity (overdose) requiring emergency management
- Neonatal abstinence syndrome in infants born to heroin-using mothers
- Infectious complications of intravenous drug use (endocarditis, abscesses, HIV, hepatitis C)
Pharmacokinetics
Absorption and bioavailability: Heroin can be administered intravenously, intranasally (snorted), or by inhalation (smoked). Intravenous administration produces effects within seconds, while intranasal and inhalation routes produce effects within minutes. Oral bioavailability is poor due to extensive first-pass metabolism.
Distribution: Heroin is highly lipophilic and crosses the blood-brain barrier rapidly—faster than morphine—which contributes to its intense euphoric effects and high abuse potential. It distributes widely into tissues.
Metabolism: Heroin undergoes rapid hydrolysis by plasma and tissue esterases. The half-life of heroin itself is only 2-6 minutes. It is first deacetylated to 6-monoacetylmorphine (6-MAM), which has a half-life of approximately 30 minutes, and then to morphine (half-life 2-3 hours). Morphine is further metabolized in the liver via glucuronidation to morphine-3-glucuronide (inactive) and morphine-6-glucuronide (active analgesic).
Excretion: Metabolites are primarily excreted in urine. Detection of 6-MAM in urine is specific for heroin use, as it is not produced by metabolism of prescription opioids. Morphine and its glucuronides can be detected in urine for 1-3 days after heroin use.
Duration of action: The euphoric effects last 3-5 hours, but the duration depends on route of administration and individual tolerance.
Adverse effects
Acute toxicity (overdose)
The opioid toxidrome consists of:
- Respiratory depression: decreased respiratory rate, hypoxia, apnea—the leading cause of death
- Central nervous system depression: sedation, stupor, coma
- Miosis: pinpoint pupils (classic triad with CNS and respiratory depression)
Other acute effects include bradycardia, hypotension, hypothermia, decreased bowel sounds, pulmonary edema, and seizures (more common with hypoxia or co-ingestion).
Chronic use complications
- Addiction and dependence: compulsive drug-seeking, tolerance, withdrawal syndrome
- Infections: HIV, hepatitis B and C, bacterial endocarditis, skin and soft tissue abscesses, septic thrombophlebitis, osteomyelitis, septic arthritis
- Pulmonary: aspiration pneumonia, pulmonary edema, talc granulomatosis (from injecting crushed tablets)
- Renal: glomerulonephritis (often associated with chronic infections)
- Neurologic: toxic leukoencephalopathy (from adulterants), hypoxic brain injury
Withdrawal syndrome
Onset 6-12 hours after last use, peaks at 24-48 hours, lasts 5-10 days. Symptoms include:
- Dysphoria, anxiety, insomnia, irritability
- Lacrimation, rhinorrhea, yawning
- Mydriasis (dilated pupils)
- Piloerection ("cold turkey")
- Diaphoresis, chills
- Nausea, vomiting, diarrhea, abdominal cramping
- Muscle aches, bone pain
- Tachycardia, hypertension
While extremely uncomfortable, opioid withdrawal is rarely life-threatening in otherwise healthy adults, unlike alcohol or benzodiazepine withdrawal.
Neonatal abstinence syndrome
Infants born to heroin-using mothers may develop withdrawal symptoms 24-72 hours after birth, including irritability, high-pitched crying, tremors, hypertonia, feeding difficulties, vomiting, diarrhea, and seizures.
Contraindications and warnings
Absolute contraindications (in countries where diamorphine is used medically):
- Known hypersensitivity to heroin or morphine
- Acute respiratory depression or severe respiratory disease without ventilatory support
- Paralytic ileus
- Concurrent or recent (within 14 days) monoamine oxidase inhibitor use
Warnings and precautions:
- Pregnancy: Heroin crosses the placenta. Chronic use during pregnancy causes fetal dependence and neonatal abstinence syndrome. Associated with increased risk of preterm birth, intrauterine growth restriction, placental abruption, fetal death, and neonatal complications. Opioid maintenance therapy (methadone or buprenorphine) is preferred to continued heroin use.
- Overdose risk: Dramatically increased after periods of abstinence (detoxification, incarceration, hospitalization) due to loss of tolerance. Contamination with fentanyl or fentanyl analogs has become a leading cause of overdose deaths.
- Infectious disease screening: All patients with heroin use disorder should be screened for HIV, hepatitis B and C, and tuberculosis.
- Co-intoxication: Concurrent use of alcohol, benzodiazepines, or other CNS depressants dramatically increases overdose risk.
- Adulterants: Street heroin may be cut with various substances including fentanyl, carfentanil, xylazine, quinine, or other drugs, increasing unpredictability and toxicity.
Drug interactions
CNS depressants (alcohol, benzodiazepines, barbiturates, sedative-hypnotics, other opioids): Additive respiratory and CNS depression; greatly increased risk of fatal overdose. This is the most clinically significant interaction.
Monoamine oxidase inhibitors (MAOIs): Can cause severe and unpredictable potentiation of opioid effects, including respiratory depression, hypotension, and serotonin syndrome. Avoid concurrent use.
CYP3A4 inhibitors (azole antifungals, macrolide antibiotics, protease inhibitors): May decrease metabolism of morphine (the active metabolite), potentially increasing effects and toxicity.
CYP3A4 inducers (rifampin, carbamazepine, phenytoin): May increase metabolism of morphine, potentially reducing analgesic effects in those using diamorphine medically.
Serotonergic drugs (SSRIs, SNRIs, MAOIs, tramadol, linezolid): Theoretical risk of serotonin syndrome, though less commonly reported with pure mu-agonist opioids.
Naltrexone and naloxone: Opioid antagonists that block the effects of heroin. Naltrexone is used for relapse prevention in abstinent patients. Naloxone is the antidote for acute overdose.
Anticholinergic drugs: May increase risk of constipation and urinary retention.
Antiretroviral therapy: Important consideration as many patients with heroin use disorder have HIV. Drug-drug interactions must be evaluated, and adherence to antiretroviral therapy may be compromised by active substance use.
Clinical pearls
Overdose management
Naloxone (Narcan) is the specific antidote for opioid toxicity. Initial dose is typically given intranasally, intramuscularly, or intravenously. Because naloxone has a shorter half-life (30-90 minutes) than heroin's active metabolites, repeated doses or continuous infusion may be necessary, and patients require monitoring for re-sedation. Naloxone administration may precipitate acute withdrawal in dependent patients but is life-saving and should never be withheld.
Harm reduction
Naloxone distribution programs provide take-home naloxone kits to patients and their families to reverse overdoses before emergency services arrive. Supervised injection facilities and fentanyl test strips are harm reduction strategies used in some jurisdictions. Needle exchange programs reduce transmission of blood-borne infections.
Treatment of opioid use disorder
Medication-assisted treatment (MAT) with methadone or buprenorphine is the gold standard and significantly reduces mortality, criminal activity, and infectious disease transmission. Methadone is a long-acting full mu-agonist; buprenorphine is a partial agonist with a ceiling effect on respiratory depression. Naltrexone (an antagonist) can be used after detoxification for relapse prevention. Behavioral therapies and psychosocial support are important adjuncts.
Urine drug screening
Heroin is metabolized to morphine, so immunoassay screens typically detect opiates. 6-Monoacetylmorphine (6-MAM) is specific for heroin use and can be detected by confirmatory testing (GC-MS) but has a short detection window (6-8 hours). Morphine can also result from use of prescription morphine or codeine (which is metabolized to morphine), so 6-MAM is the definitive marker.
Fentanyl contamination
The illicit drug supply is increasingly contaminated with synthetic opioids, particularly fentanyl and analogs, which are 50-100 times more potent than morphine. This has led to a dramatic increase in overdose deaths. Clinicians should counsel patients on this risk and ensure access to naloxone.
Infectious endocarditis
Intravenous heroin use is a major risk factor for right-sided endocarditis, typically involving the tricuspid valve and caused by Staphylococcus aureus. Presentation includes fever, septic pulmonary emboli (multiple nodular infiltrates on chest X-ray), and often absence of a heart murmur initially.
Opioid toxidrome: COMA — CNS depression, Opiates (or Overdose), Miosis, Apnea (respiratory depression). Remember naloxone for reversal.
Practice questions
1. A 28-year-old man is brought to the emergency department by ambulance after being found unresponsive in a public restroom. On arrival, he is obtunded with minimal response to painful stimuli. Vital signs show temperature 36.0°C (96.8°F), blood pressure 90/60 mmHg, pulse 52/min, and respirations 6/min and shallow. Physical examination reveals bilateral pinpoint pupils and track marks on both forearms. Which of the following is the most appropriate immediate intervention?
2. A 32-year-old woman with a history of intravenous heroin use presents to the clinic requesting help with stopping drug use. She last used heroin 18 hours ago and is experiencing muscle aches, rhinorrhea, lacrimation, nausea, and severe anxiety. Vital signs show temperature 37.2°C (99.0°F), blood pressure 148/92 mmHg, pulse 104/min, and respirations 18/min. Physical examination reveals dilated pupils, piloerection, and diaphoresis. Which of the following is the most appropriate long-term management?
3. A 35-year-old man with known intravenous heroin use presents to the emergency department with 3 weeks of fevers, chills, and progressive shortness of breath. Temperature is 39.1°C (102.4°F), blood pressure 110/70 mmHg, pulse 112/min, and respirations 24/min. Cardiac examination reveals no murmur. Chest X-ray shows multiple peripheral nodular infiltrates, some with cavitation. Blood cultures grow methicillin-sensitive Staphylococcus aureus. Which of the following is the most likely diagnosis?
Frequently asked questions
How does heroin work in the brain?
Heroin rapidly crosses the blood-brain barrier and is converted to 6-monoacetylmorphine and morphine, which act as potent mu-opioid receptor agonists. This activates reward pathways in the brain, releasing dopamine and producing intense euphoria, while simultaneously depressing respiratory centers in the brainstem. The rapid onset and intensity of effects contribute to its extremely high addiction potential.
What are the signs of a heroin overdose?
The classic opioid toxidrome consists of three main features: severe respiratory depression (slow, shallow breathing or apnea), central nervous system depression (stupor or coma), and pinpoint pupils (miosis). Other signs include bradycardia, hypotension, hypothermia, cyanosis, and decreased bowel sounds. Respiratory depression is the primary cause of death in heroin overdose. Immediate treatment with naloxone can reverse these effects and is life-saving.
Is heroin a controlled substance?
Yes, heroin is a Schedule I controlled substance in the United States, meaning it has high abuse potential and no accepted medical use. In the UK and some other countries, pharmaceutical diamorphine (medical-grade heroin) is a controlled substance but is used legally for severe pain management in hospital settings. However, street heroin is illegal worldwide and carries significant legal penalties for possession, distribution, and trafficking.
What is the difference between heroin and morphine?
Heroin (diacetylmorphine) is a semi-synthetic derivative of morphine with acetyl groups added at two positions, making it much more lipid-soluble than morphine. This allows heroin to cross the blood-brain barrier much faster, producing more rapid and intense euphoric effects. Once in the body, heroin is quickly converted back to morphine, which is the primary active compound. Morphine is a legal prescription medication for severe pain, while heroin has no approved medical use in the US.
How long does heroin withdrawal last?
Heroin withdrawal symptoms typically begin 6-12 hours after the last use, peak in intensity at 24-48 hours, and gradually subside over 5-10 days. Early symptoms include anxiety, restlessness, muscle aches, and flu-like symptoms. Later symptoms include nausea, vomiting, diarrhea, abdominal cramping, dilated pupils, and intense drug cravings. While extremely uncomfortable, opioid withdrawal is rarely life-threatening in otherwise healthy adults. However, the discomfort often drives relapse, making medication-assisted treatment important.
Conditions it is used for
Related drugs
Sources and further reading
DailyMed (FDA labels)StatPearls (NCBI Bookshelf)MedlinePlusWHO ATC/DDD index: N07BC06
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.