Mechanism of action
Hydroxychloroquine is a weak base that accumulates in acidic intracellular compartments, particularly lysosomes, raising their pH. This alkalinization interferes with multiple cellular processes including antigen processing and presentation, reducing MHC class II-mediated T cell activation.
In malaria, the drug concentrates in the parasite's digestive vacuole where it inhibits heme polymerization. Plasmodium species digest hemoglobin to obtain amino acids, releasing toxic free heme that is normally detoxified by polymerization into hemozoin. Hydroxychloroquine prevents this detoxification, leading to parasite death from heme toxicity.
The immunomodulatory effects involve inhibition of toll-like receptor (TLR) signaling, particularly TLR7 and TLR9, which recognize nucleic acids. By blocking TLR activation in plasmacytoid dendritic cells, hydroxychloroquine reduces production of interferon-alpha and other proinflammatory cytokines. It also inhibits cytokine production by T cells and B cell activation. These mechanisms underlie its efficacy in autoimmune diseases like systemic lupus erythematosus and rheumatoid arthritis.
- Hydroxychloroquine accumulates in lysosomes and raises pH, interfering with antigen presentation
- Most serious adverse effect is irreversible bull's eye maculopathy; requires annual ophthalmologic screening after 5 years
- Indicated for malaria prophylaxis (chloroquine-sensitive species), SLE, and rheumatoid arthritis
- Increases digoxin levels and can prolong QT interval, especially with azithromycin
- Safe in pregnancy and recommended to continue in SLE patients to prevent flares
- Very long half-life (40-50 days) means steady state takes months and effects persist after stopping
- Contraindicated in G6PD deficiency due to hemolysis risk
- Maximum dose is 5 mg/kg/day based on actual body weight to minimize retinal toxicity
Indications
Malaria: Hydroxychloroquine is used for prophylaxis and treatment of malaria caused by chloroquine-sensitive Plasmodium species. It is effective against P. vivax, P. ovale, P. malariae, and some strains of P. falciparum. Widespread chloroquine resistance in P. falciparum limits its use in many endemic areas. The drug does not eliminate hypnozoites and must be combined with primaquine for radical cure of P. vivax and P. ovale.
Systemic lupus erythematosus: Hydroxychloroquine is considered standard therapy for all SLE patients unless contraindicated. It reduces disease activity, prevents flares, improves long-term outcomes, and decreases damage accumulation. Benefits include reduced thrombotic risk, improved lipid profiles, and better pregnancy outcomes.
Rheumatoid arthritis: Used as a disease-modifying antirheumatic drug (DMARD), often in combination with methotrexate and sulfasalazine (triple therapy). It is particularly useful in mild to moderate disease and provides additional cardiovascular protection.
Other autoimmune conditions: Hydroxychloroquine is used off-label for discoid lupus, Sjögren syndrome, cutaneous sarcoidosis, and certain forms of chronic urticaria. It may be beneficial in antiphospholipid syndrome and Q fever endocarditis.
Pharmacokinetics
Hydroxychloroquine is well absorbed orally with bioavailability of approximately 70-80%. Peak plasma concentrations occur 3-4 hours after administration. Taking the drug with food minimizes gastrointestinal upset without significantly affecting absorption.
The volume of distribution is extremely large (5000-15000 L) due to extensive tissue binding. The drug concentrates in tissues with high melanin content including eyes, skin, and hair, as well as in kidneys, liver, lungs, and blood cells. This extensive distribution contributes to a very long elimination half-life of 40-50 days, meaning steady state is not reached for several months.
Metabolism occurs in the liver via CYP2C8, CYP3A4, and CYP2D6 to active metabolites including desethylhydroxychloroquine. Both parent drug and metabolites are eliminated primarily by renal excretion (15-25% unchanged), with some biliary excretion. Dose adjustment is required in severe renal impairment but not typically in mild to moderate disease. The prolonged half-life means drug effects and toxicity can persist for months after discontinuation.
Adverse effects
Retinal toxicity: The most serious adverse effect is irreversible retinopathy presenting as bull's eye maculopathy. Risk increases with cumulative dose (especially >1000 g total), duration of use (>5 years), renal disease, and concurrent tamoxifen use. Early changes include parafoveal thinning on optical coherence tomography before visual symptoms appear. Annual screening with automated visual fields and spectral domain OCT is recommended after 5 years of therapy. Vision loss can progress even after drug discontinuation.
Gastrointestinal: Nausea, vomiting, diarrhea, and abdominal cramping are common, occurring in 10-30% of patients. Taking the medication with food reduces these effects. Symptoms usually improve with continued therapy.
Dermatologic: Skin hyperpigmentation (blue-gray discoloration), pruritus, photosensitivity, and hair bleaching or loss can occur. Exacerbation of psoriasis is well-recognized.
Cardiovascular: Cardiomyopathy is rare but serious, presenting with conduction abnormalities, hypertrophy, or heart failure. QT prolongation may occur, particularly with high doses or in combination with other QT-prolonging drugs.
Hematologic: Rare cases of aplastic anemia, agranulocytosis, and thrombocytopenia have been reported. Hemolysis can occur in patients with glucose-6-phosphate dehydrogenase (G6PD) deficiency.
Neuropsychiatric: Headache, dizziness, nervousness, and emotional changes are reported. Rare toxic psychosis and seizures may occur.
Myopathy: Skeletal or cardiac myopathy manifesting as proximal muscle weakness with elevated creatine kinase can develop, particularly with prolonged use.
Contraindications and warnings
Absolute contraindications: Known hypersensitivity to hydroxychloroquine or 4-aminoquinolines, pre-existing retinopathy or maculopathy, and G6PD deficiency (relative contraindication due to hemolysis risk).
Pregnancy and lactation: Hydroxychloroquine is pregnancy category C but is generally considered safe and is recommended to be continued during pregnancy in patients with SLE and rheumatoid arthritis. The benefits of disease control outweigh theoretical risks, and discontinuation may precipitate disease flares. The drug is excreted in breast milk in small amounts but is considered compatible with breastfeeding.
Hepatic and renal impairment: Use with caution in hepatic disease as the drug is metabolized in the liver. Dose reduction is recommended in severe renal impairment (GFR <10 mL/min) due to renal elimination.
Cardiac disease: Caution is advised in patients with heart disease, particularly those with conduction disorders or QT prolongation. Baseline ECG should be considered in high-risk patients.
Dermatologic conditions: Hydroxychloroquine may exacerbate psoriasis and porphyria. Screen for these conditions before initiating therapy.
Monitoring: Baseline ophthalmologic examination is recommended before starting therapy. Annual screening should begin after 5 years of use (or earlier if risk factors present). Periodic complete blood counts should be performed to detect rare hematologic toxicity.
Drug interactions
QT-prolonging drugs: Concurrent use with azithromycin, fluoroquinolones, antiarrhythmics (amiodarone, sotalol), antipsychotics, and other QT-prolonging agents increases risk of ventricular arrhythmias including torsades de pointes. This combination gained attention during COVID-19 when hydroxychloroquine was used with azithromycin.
Digoxin: Hydroxychloroquine increases serum digoxin levels by reducing renal clearance, potentially causing digoxin toxicity. Monitor digoxin concentrations and adjust dose accordingly.
Antacids and kaolin: These agents can reduce hydroxychloroquine absorption. Separate administration by at least 4 hours.
Antiepileptic drugs: Hydroxychloroquine may lower the seizure threshold and reduce efficacy of antiepileptic medications, increasing breakthrough seizure risk.
Methotrexate: When used together for rheumatoid arthritis (common combination), there is no significant pharmacokinetic interaction, but additive immunosuppression may increase infection risk.
Tamoxifen: Concurrent use increases risk of retinopathy beyond that of either drug alone. Avoid this combination when possible.
Insulin and oral hypoglycemics: Hydroxychloroquine may enhance hypoglycemic effects, requiring adjustment of antidiabetic medication doses. This effect may actually be beneficial in diabetic patients with lupus or rheumatoid arthritis.
CYP2D6 substrates: As a CYP2D6 inhibitor, hydroxychloroquine can increase levels of drugs metabolized by this enzyme including metoprolol, propranolol, and some antidepressants.
Clinical pearls
Dosing is weight-based: Calculate dose based on ideal body weight, not actual body weight, to reduce toxicity risk. The recommended dose for rheumatologic conditions is ≤5 mg/kg/day of actual body weight (≤400 mg/day for most patients).
Delayed onset of action: In autoimmune diseases, therapeutic effects may take 6-12 weeks to become apparent. Patients should be counseled about this delayed benefit to improve adherence.
Pregnancy in SLE: Hydroxychloroquine should be continued throughout pregnancy in lupus patients. Discontinuation increases flare risk, which poses greater danger to mother and fetus than continued drug exposure.
Screening timing: The American Academy of Ophthalmology recommends baseline screening, then annual screening after 5 years. High-risk patients (renal disease, tamoxifen use, daily dose >5 mg/kg) should begin screening earlier.
Not for chloroquine-resistant malaria: Geographic resistance patterns must guide malaria prophylaxis choices. P. falciparum in most of Africa, Southeast Asia, and South America is chloroquine-resistant.
Lower cardiovascular events: In observational studies of patients with SLE and rheumatoid arthritis, hydroxychloroquine use is associated with reduced cardiovascular events and improved lipid profiles, adding to its disease-modifying benefits.
Drug levels: Therapeutic drug monitoring is not routinely performed but can be useful to assess adherence. Whole blood levels >750 ng/mL are associated with therapeutic efficacy in SLE.
Exam tip: A USMLE question presenting a patient with rheumatoid arthritis or lupus on long-term hydroxychloroquine who develops visual changes should make you think of irreversible retinopathy, not just common cataracts.
Practice questions
1. A 35-year-old woman with systemic lupus erythematosus has been taking hydroxychloroquine 400 mg daily for 7 years. She presents for a routine follow-up and mentions no visual complaints. Which of the following is the most appropriate next step in monitoring for drug toxicity?
2. A 45-year-old man with rheumatoid arthritis on hydroxychloroquine 400 mg daily presents to the emergency department with fever and cough. His ECG shows QT interval of 440 ms. The physician plans to start empiric antibiotic therapy. Which medication should be avoided due to risk of interaction with hydroxychloroquine?
3. A 28-year-old woman with lupus who is 8 weeks pregnant asks whether she should stop taking hydroxychloroquine due to concerns about fetal harm. Her lupus has been well-controlled on hydroxychloroquine 400 mg daily for 2 years. What is the most appropriate recommendation?
Frequently asked questions
How does hydroxychloroquine work?
Hydroxychloroquine is a lysosomotropic agent that accumulates in acidic cellular compartments and raises their pH. In malaria, it prevents heme detoxification in the parasite's digestive vacuole, causing toxic heme accumulation. In autoimmune diseases, it inhibits antigen presentation, toll-like receptor signaling (especially TLR7 and TLR9), and reduces interferon-alpha and other cytokine production, thereby dampening immune activation.
What are the side effects of hydroxychloroquine?
The most serious side effect is irreversible retinal toxicity presenting as bull's eye maculopathy, especially with cumulative doses over 1000 grams or use exceeding 5 years. Common side effects include gastrointestinal upset (nausea, diarrhea), skin hyperpigmentation, and photosensitivity. Rare but serious effects include cardiomyopathy with conduction abnormalities, QT prolongation, blood dyscrasias, and myopathy. Annual eye exams are required for long-term users.
Is hydroxychloroquine safe during pregnancy?
Yes, hydroxychloroquine is considered safe during pregnancy and is actually recommended to be continued in pregnant women with systemic lupus erythematosus and rheumatoid arthritis. Discontinuing the drug increases the risk of disease flares, which pose greater danger to both mother and fetus than continued drug exposure. Studies have not shown increased risk of birth defects. It is also compatible with breastfeeding.
What is the difference between hydroxychloroquine and chloroquine?
Hydroxychloroquine is a hydroxylated derivative of chloroquine with similar antimalarial and immunomodulatory mechanisms. The key difference is that hydroxychloroquine has a better safety profile with lower risk of retinal toxicity, making it preferred for long-term use in autoimmune diseases. Both drugs share the same resistance patterns in malaria and have similar efficacy, but hydroxychloroquine's improved tolerability makes it the standard choice for rheumatologic conditions.
Why do patients on hydroxychloroquine need eye exams?
Hydroxychloroquine can cause irreversible retinal toxicity, particularly affecting the macula and producing a characteristic bull's eye appearance on fundoscopy. This damage is dose and duration dependent, with risk increasing significantly after 5 years of use or cumulative doses exceeding 1000 grams. Early detection through screening with visual field testing and optical coherence tomography (OCT) allows discontinuation before severe vision loss occurs, though some progression may continue even after stopping the drug.
Conditions it is used for
Related drugs
Sources and further reading
DailyMed (FDA labels)StatPearls (NCBI Bookshelf)MedlinePlusWHO ATC/DDD index: P01BA02
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.