Hydroxychloroquine is generally well tolerated at recommended doses, which is why it remains a cornerstone treatment for lupus, rheumatoid arthritis, and other autoimmune conditions. But “well tolerated” does not mean risk-free. A small percentage of people on the drug develop serious complications affecting the eyes, heart, muscles, blood, or skin, and some of these problems can become irreversible if they go undetected. The risks climb with higher doses, longer use, and certain genetic profiles, which makes understanding them more than academic.
Retinal Toxicity and Vision Loss
The side effect that gets the most attention from specialists is retinal toxicity. Long-term hydroxychloroquine use can damage the macula, the part of the retina responsible for sharp central vision, and once the damage reaches a certain point it cannot be reversed.1PubMed Central. Hydroxychloroquine-induced Retinal Toxicity The risk stays low during the first several years of treatment but climbs sharply toward about one percent after five to seven years, or after a cumulative lifetime dose of around 1,000 grams.2PubMed. Revised recommendations on screening for chloroquine and hydroxychloroquine retinopathy With continued use beyond that window, the likelihood keeps rising.
What makes this particularly tricky is that early retinal damage is silent. You won’t notice any change in your vision until a significant amount of tissue has been affected. To catch it before symptoms appear, the American Academy of Ophthalmology recommends a baseline eye exam when you start the drug, then annual screening after five years of use. Patients with additional risk factors, such as kidney disease that slows the drug’s clearance, should start screening sooner.2PubMed. Revised recommendations on screening for chloroquine and hydroxychloroquine retinopathy The current dosing guideline caps hydroxychloroquine at 5 mg per kilogram of actual body weight per day to limit this risk, yet one study at a large academic medical center found that a third of patients were prescribed doses exceeding that threshold.3PubMed Central. Compliance With Hydroxychloroquine Screening Guidelines at a Large Academic Medical Center
Modern screening relies on optical coherence tomography (OCT), which can detect thinning of the retinal layers before traditional visual field tests pick up anything wrong. In a study of patients on long-term hydroxychloroquine, most showed slow, stable thinning of the retina averaging well under one micrometer per year. But a subset showed a period of rapid thinning, losing several micrometers per year, and many of those patients went on to develop conventional signs of toxicity.4PubMed. Rapid Macular Thinning Is an Early Indicator of Hydroxychloroquine Retinal Toxicity That rapid-thinning phase may be an important early warning: catching it could allow a doctor to stop or reduce the drug before irreversible vision loss sets in. Meanwhile, another layer of the retina, the ganglion cell layer, also thins in proportion to the total cumulative dose of the drug, providing an additional measurable signal for screening.5PubMed Central. A New Objective Parameter in Hydroxychloroquine-Induced Retinal Toxicity Screening Test: Macular Retinal Ganglion Cell-Inner Plexiform Layer Thickness
Heart Rhythm Problems and Cardiomyopathy
Hydroxychloroquine can affect the heart’s electrical system, prolonging a measurement on the electrocardiogram called the QTc interval. When the QTc stretches too far, it raises the risk of a dangerous heart rhythm disturbance called torsades de pointes, which can cause fainting or cardiac arrest.6PubMed Central. Hydroxychloroquine, QTc prolongation and risk of torsades de pointes People who already have a longer-than-normal QTc, structural heart disease, electrolyte imbalances, or kidney problems face heightened danger. Taking other medications that also prolong the QTc, such as certain antibiotics, compounds the risk further. During the early months of the COVID-19 pandemic, when hydroxychloroquine was frequently paired with azithromycin (another QTc-prolonging drug), roughly one in five treated patients showed a prolonged QTc interval after treatment, and two cases of torsades de pointes occurred in a single study cohort.7PubMed Central. Effect of Hydroxychloroquine and Azithromycin on QT Interval Prolongation and Other Cardiac Arrhythmias in COVID-19 Confirmed Patients
Beyond rhythm disturbances, long-term use of hydroxychloroquine has been linked to cardiomyopathy, where the heart muscle itself weakens. The drug accumulates inside lysosomes, the cell’s recycling compartments, and disrupts enzymes that normally break down waste material. Over time, this can lead to a buildup of toxic material in heart muscle cells.8PubMed Central. Hydroxychloroquine-Induced Cardiomyopathy: A Case Report Hydroxychloroquine-induced cardiomyopathy is rare enough that it often goes unrecognized at first, sometimes being confused with other causes of heart failure. When caught, stopping the drug can halt progression, though recovery is not guaranteed.
Muscle Weakness and Myopathy
The same lysosomal accumulation mechanism that damages heart muscle can affect skeletal muscle, producing a condition called vacuolar myopathy. Patients typically develop progressive weakness in the muscles closest to the trunk, like the thighs and upper arms, making it hard to climb stairs or lift objects overhead. Muscle biopsies in affected patients show characteristic abnormalities consistent with hydroxychloroquine toxicity, including disruption of the cell’s energy-producing structures.9PubMed. Hydroxychloroquine-induced autophagic vacuolar myopathy with mitochondrial abnormalities This side effect can be especially confusing because autoimmune diseases like lupus can themselves cause muscle inflammation. The key distinguishing clue is often timing: if weakness worsens while the underlying disease seems otherwise controlled, the drug itself should be suspected.
Stevens-Johnson Syndrome
Hydroxychloroquine very rarely triggers Stevens-Johnson syndrome (SJS), a severe skin reaction in which the outer layer of the skin blisters and peels away from the underlying tissue. SJS is a medical emergency that can affect the mouth, eyes, and internal organs, and it carries a real risk of death. Hydroxychloroquine is considered a very rare cause of drug-induced SJS.10PubMed Central. Hydroxychloroquine-induced Stevens–Johnson syndrome in COVID-19: a rare case report The handful of reported cases include patients with lupus as well as patients who received the drug during the COVID-19 pandemic.11PubMed Central. Hydroxychloroquine-Induced Stevens-Johnson Syndrome in the Patient with Systemic Lupus Erythematosus: A Case Report in Kurdish Region – Iraq In one case series, all 11 patients who developed SJS had been taking hydroxychloroquine, some as treatment for COVID-19 and others as prophylaxis.12Annals of Medical and Health Sciences Research. Stevens Johnson Syndrome in Association with Hydroxychloroquine Treatment for Covid 19 A Case Series Because SJS can be fatal, any new widespread rash with blistering, mouth sores, or eye irritation while on the drug warrants immediate medical attention.
Blood Cell Disorders
Hydroxychloroquine can, on rare occasions, suppress the production of white blood cells or trigger the destruction of red blood cells. Agranulocytosis, a severe drop in a specific type of white blood cell called neutrophils, leaves the body dangerously vulnerable to infections. In one reported case, a woman with rheumatoid arthritis developed agranulocytosis after several months on the drug; her blood counts returned to normal within two weeks of stopping it.13PubMed Central. Hydroxychloroquine-induced agranulocytosis in a patient with long-term rheumatoid arthritis
A different blood-related risk affects people with glucose-6-phosphate dehydrogenase (G6PD) deficiency, an inherited enzyme shortage that is particularly common in men of African, Mediterranean, and Southeast Asian descent. In these individuals, hydroxychloroquine can trigger hemolytic anemia, where red blood cells are destroyed faster than the body can replace them. A case report documented acute hemolytic anemia in a man with G6PD deficiency who completed a standard five-day course of the drug for COVID-19.14PubMed Central. Hemolytic Anemia in a Glucose-6-Phosphate Dehydrogenase-Deficient Patient Receiving Hydroxychloroquine for COVID-19: A Case Report Some clinicians recommend G6PD testing before prescribing the drug, though this is not yet universal practice.
Dangerously Low Blood Sugar
Hydroxychloroquine can lower blood sugar, a side effect that tends to catch people off guard because it can occur even in patients who do not have diabetes. The drug appears to alter how the body processes insulin, reducing insulin resistance and slowing insulin breakdown inside cells.15Journal of Rheumatic Diseases and Treatment. A Case of Hydroxychloroquine Induced Hypoglycaemia in a Non-Diabetic Patient For people who are already on diabetes medications like insulin or sulfonylureas, adding hydroxychloroquine can push blood sugar dangerously low. But hypoglycemia has also been documented in people with no diabetes at all.16PubMed. Hydroxychloroquine-Induced Hypoglycemia in a Patient Without Diabetes Symptoms include shakiness, confusion, sweating, and in severe cases, loss of consciousness. If you are on hydroxychloroquine and notice unexplained episodes of feeling faint or confused, especially between meals, hypoglycemia is worth checking for.
Hearing Loss
Hydroxychloroquine can damage the inner ear, causing sensorineural hearing loss, the type that results from harm to the delicate hair cells or nerves of the cochlea rather than a mechanical blockage. A systematic review compiling available evidence found that the drug has an ototoxic effect on the auditory system, with hearing loss varying in severity and affecting one or both ears.17PubMed Central. The Ototoxicity of Chloroquine and Hydroxychloroquine: A Systematic Review Unlike conductive hearing loss, sensorineural damage is generally permanent. This side effect is not well known among patients, and it probably goes underreported because hearing changes are easy to attribute to aging or noise exposure.
Neuropsychiatric Symptoms
Reports of psychiatric side effects from hydroxychloroquine include insomnia, vivid dreams, anxiety, hallucinations, and, in rare instances, psychosis or suicidal behavior. A pharmacovigilance analysis of over 1,700 COVID-19 patients treated with the drug found 56 psychiatric adverse events, half of which were classified as serious. These included suicides, self-injury, and psychotic episodes with hallucinations or agitation.18Primary Care Companion for CNS Disorders. Psychiatric Adverse Effects From Hydroxychloroquine Use: A Systematic Review
However, disentangling drug effects from the underlying disease is difficult. A large multinational study compared rheumatoid arthritis patients starting hydroxychloroquine against those starting a different drug and found no consistent increase in the risk of depression, suicidal ideation, or acute psychosis.19PubMed Central. Risk of depression, suicide and psychosis with hydroxychloroquine treatment for rheumatoid arthritis: a multinational network cohort study The meta-analytic estimates for all three outcomes hovered near one, meaning no measurable difference between groups. The apparent contradiction between individual case reports and large cohort data suggests that psychiatric reactions, if they are genuinely drug-caused, are extremely rare and may depend on individual vulnerability rather than a general drug effect. Still, patients and clinicians should be aware of the possibility, and any sudden personality changes or unusual psychological symptoms while on the drug deserve a conversation with a prescriber.
Acute Overdose
The risks described above typically emerge during long-term therapeutic use. Acute overdose is a different scenario entirely and is life-threatening. In overdose, the cardiac effects accelerate dramatically: the heart’s conduction system can shut down in a cascade from a single blocked pathway to complete heart block, accompanied by dangerously low blood pressure from the drug’s effect on blood vessels. QTc intervals in reported overdose cases have stretched far beyond normal, and fatal arrhythmias can follow.20PubMed Central. Acute chloroquine and hydroxychloroquine toxicity: A review for emergency clinicians One case report described a woman who ingested 20 grams of hydroxychloroquine and developed ventricular arrhythmias requiring aggressive treatment with multiple medications to stabilize her heart rhythm.21PubMed Central. Nearly Fatal Hydroxychloroquine Overdose Successfully Treated with Midazolam, Propofol, Sodium Bicarbonate, Norepinephrine, and Intravenous Lipid Emulsion There is no specific antidote, so emergency treatment focuses on supporting heart rhythm and blood pressure while the drug is slowly cleared from the body.
Why Some People Are More Vulnerable
Not everyone metabolizes hydroxychloroquine at the same rate. The drug is broken down primarily by a family of liver enzymes, and genetic variations in the genes encoding those enzymes can make a person process the drug much faster or much slower than average. Both extremes create problems. People who break the drug down too slowly end up with higher blood levels, raising the risk of toxicity. People who metabolize it too quickly may not reach therapeutic levels, leading to treatment failure and potentially unnecessary dose increases that then overshoot.22PubMed. Pharmacogenomics of chloroquine and hydroxychloroquine: current evidence and future implications
Research has identified specific gene variants that influence drug levels. Variations in one gene, CYP2D6, affect how quickly the drug is converted to its active breakdown product, and those variations are distributed unevenly across different ethnic populations.23PubMed. Association of Polymorphisms of Cytochrome P450 2D6 With Blood Hydroxychloroquine Levels in Patients With Systemic Lupus Erythematosus Other variants in genes called CYP3A4 and CYP2C8 have been linked to differences in blood levels of the drug and to specific types of adverse reactions, including eye problems, skin reactions, and kidney effects.24PubMed Central. Relationship of cytochrome P450 gene polymorphisms with blood concentrations of hydroxychloroquine and its metabolites and adverse drug reactions Pharmacogenomic testing before starting hydroxychloroquine is not yet standard practice, but the growing evidence suggests that a substantial fraction of patients carry gene variants that put them at higher risk of either ineffective treatment or serious side effects.
Hydroxychloroquine During Pregnancy
For women with lupus, stopping hydroxychloroquine during pregnancy risks disease flares that themselves threaten the pregnancy. So the question is not simply whether the drug is “safe” for a fetus, but whether the risk of the drug outweighs the risk of uncontrolled disease. The evidence here is somewhat reassuring but not entirely clean. A large cohort study found that the overall rate of major birth defects was modestly higher in hydroxychloroquine-exposed pregnancies than in unexposed ones, but the increase was concentrated in women taking daily doses of 400 mg or more. At lower doses, no increase was detected.25PubMed Central. Hydroxychloroquine early in pregnancy and risk of birth defects
A separate prospective study compared women who took hydroxychloroquine in the first trimester to disease-matched controls who did not, and found no significant difference in rates of major birth defects, spontaneous abortion, or preterm delivery. The one measurable difference was a slightly smaller head circumference at birth in the exposed group, though even that finding was modest.26PubMed. Birth Outcomes in Women Who Have Taken Hydroxycholoroquine During Pregnancy: A Prospective Cohort Study Taken together, these studies suggest that hydroxychloroquine at moderate doses does not pose a major risk to the developing baby, but that higher doses warrant extra scrutiny. For most women with lupus or related conditions, rheumatologists continue to recommend staying on the drug during pregnancy because the alternative, disease flares, carries its own fetal risks.
Drug Interactions That Amplify Risk
Hydroxychloroquine’s cardiac effects become more dangerous when combined with other drugs that prolong the QTc interval. Common offenders include certain antibiotics (azithromycin, fluoroquinolones), some antidepressants, antipsychotics, and anti-nausea medications. During the COVID-19 pandemic, the combination of hydroxychloroquine and azithromycin became widespread before adequate safety data existed. In one study, about a quarter of patients on the combination developed a prolonged QTc interval after treatment, compared to about ten percent of those on hydroxychloroquine alone, though the difference did not reach statistical significance in that particular sample.7PubMed Central. Effect of Hydroxychloroquine and Azithromycin on QT Interval Prolongation and Other Cardiac Arrhythmias in COVID-19 Confirmed Patients Male sex and having an already-elevated baseline QTc were independent risk factors for developing dangerous prolongation.
Beyond heart rhythm interactions, hydroxychloroquine’s blood-sugar-lowering effect can combine unpredictably with diabetes medications. If you are on insulin or sulfonylureas and your doctor adds hydroxychloroquine, you should discuss whether your diabetes regimen needs adjusting. The same principle applies to drugs that affect the liver enzymes responsible for metabolizing hydroxychloroquine. Strong inhibitors of those enzymes could raise drug levels, while strong inducers could lower them, potentially moving you outside the safe therapeutic window without any dose change on paper.
Practical Screening and Monitoring
The weight-based dosing guideline of no more than 5 mg per kilogram of actual body weight per day is the single most important risk-reduction measure. It was issued specifically to limit retinal toxicity.27PubMed Central. Adherence to Weight-Based Dosing Guidelines in Patients Receiving Hydroxychloroquine for Systemic Lupus Erythematosus and Rheumatoid Arthritis: Results of a Quality Improvement Initiative Yet adherence is uneven, partly because many patients have been on fixed doses for years without recalculation. A smartphone-based dosing calculator has been developed to make weight-based dosing easier for clinicians to apply in practice.28PubMed Central. Solving the Hydroxychloroquine Dosing Dilemma With a Smartphone App
For the heart, a baseline electrocardiogram before starting the drug and periodic follow-ups help catch QTc prolongation before it becomes dangerous. For blood counts, routine monitoring can pick up agranulocytosis or anemia early. And for the often-overlooked risk of hearing loss, mentioning the possibility to your prescriber if you notice any change in hearing ensures the symptom gets evaluated rather than dismissed. None of these monitoring steps are burdensome, but they do require both the patient and the prescriber to remember that a drug can be simultaneously effective and quietly harmful over the long haul.