Hydroxychloroquine causes gastrointestinal symptoms more than any other type of side effect, with abdominal pain affecting roughly 10% of users and nausea about 6%. Most people tolerate the drug well at recommended doses, but it can affect nearly every organ system, from your eyes and heart to your blood cells and muscles. The side effects that matter most depend on how long you take it and at what dose.
Digestive Problems Are the Most Common
Stomach and intestinal symptoms are the leading reason people stop taking hydroxychloroquine. The most frequent complaints are nausea, heartburn, diarrhea, abdominal cramps, bloating, loss of appetite, and vomiting. These tend to show up early in treatment and sometimes improve over weeks as your body adjusts. Taking the medication with food or splitting the dose can help reduce stomach upset.
Skin Reactions
Hydroxychloroquine can trigger itching, rashes, hives, and increased sensitivity to sunlight. Some people develop changes in skin pigmentation, particularly with long-term use, where the skin takes on a blue-gray or brownish discoloration. Hair thinning is another possibility. In rare cases, more severe skin reactions can occur, including blistering or peeling conditions that require immediate medical attention.
Retinal Damage With Long-Term Use
The side effect that gets the most attention is toxicity to the retina, the light-sensing tissue at the back of your eye. This is uncommon in the first several years but becomes a real concern with prolonged use. In a study of 537 patients, about 1% developed retinopathy within the first 5 years. That rose to 3.3% between 11 and 15 years, and jumped to 11.5% between 16 and 20 years of continuous treatment.
The damage can be irreversible, which is why screening matters. The American Academy of Ophthalmology recommends a baseline eye exam soon after starting the drug, including an imaging scan called OCT and a test called fundus autofluorescence. Annual screening with those same tools is recommended while you’re on hydroxychloroquine, though it may be deferred during the first 5 years if you don’t have additional risk factors. The goal is to catch any retinal changes early, before they affect your vision. Blurred vision, difficulty focusing, and light sensitivity can also occur independent of retinal damage.
To reduce the risk, current guidelines recommend keeping the dose at or below 5 mg per kilogram of your actual body weight per day. This threshold is endorsed by both ophthalmology and rheumatology professional organizations.
Heart Rhythm and Structural Changes
Hydroxychloroquine can affect the heart’s electrical system, particularly by prolonging the QT interval, a measure of how long your heart takes to reset between beats. When this interval stretches too far, it raises the risk of dangerous irregular rhythms. In cases of acute overdose, QT prolongation is the most common cardiac event, sometimes progressing to life-threatening arrhythmias.
Long-term use carries a different cardiac risk profile. Rather than rhythm problems alone, years of use (typically over a decade, with very high cumulative doses) have been linked to structural damage, especially heart failure. In a systematic review, 60 cases of new left-sided heart failure were reported among chronic users, most of whom had taken the drug for more than 10 years. Heart block, where electrical signals between the upper and lower chambers of the heart are disrupted, has also been documented. These structural effects are uncommon but serious, and they tend to develop insidiously.
Muscle Weakness
Hydroxychloroquine can cause a type of muscle disease that develops gradually. The hallmark is painless weakness in the muscles closest to your trunk, like your thighs and upper arms, making it harder to climb stairs, stand from a chair, or lift objects overhead. This can be tricky to recognize because it comes on slowly and can mimic a flare of the underlying condition being treated, such as lupus. In reported cases, patients have been misdiagnosed with disease flares before the medication was identified as the cause. Muscle damage from hydroxychloroquine is generally reversible once the drug is stopped, though recovery can take time.
Neurological and Psychiatric Effects
Hydroxychloroquine crosses into the brain and acts as a central nervous system stimulant. Reported neurological side effects include headache, dizziness, fatigue, irritability, difficulty sleeping, vivid or disturbing dreams, and in rare cases, seizures or confusion. Some people experience tinnitus (ringing in the ears) or vertigo.
Psychiatric effects are less common but documented: mood instability, agitation, and rarely psychosis. In children, these effects deserve particular attention. A case report of a 6-year-old girl described restlessness, confusion, and involuntary muscle jerks after starting hydroxychloroquine at standard doses. While pediatric neuropsychiatric reactions are rare, the drug’s ability to reach the brain means they’re possible at any age, especially when combined with corticosteroids or other medications that amplify its central nervous system effects.
Low Blood Sugar
Hydroxychloroquine can cause severe drops in blood sugar, even in people who don’t have diabetes and aren’t taking diabetes medications. This effect is serious enough that it has caused loss of consciousness in some cases. If you’re on insulin or other blood sugar-lowering drugs, the combination can amplify this risk, and your doses may need adjustment.
Blood Cell Changes
In very rare cases, hydroxychloroquine reduces the production of blood cells. This can mean low white blood cell counts (raising infection risk), low platelet counts (raising bleeding risk), or anemia. Some providers check blood counts at baseline and periodically during treatment for this reason.
Drug Interactions That Increase Risk
Several medications amplify the side effects of hydroxychloroquine or are amplified by it. The combinations that pose the greatest concern involve other drugs that affect heart rhythm. Digoxin levels can rise when taken alongside hydroxychloroquine, increasing the chance of cardiac toxicity. Diabetes medications, including insulin, may cause blood sugar to drop further than expected. Antacids can reduce how well hydroxychloroquine is absorbed, making it less effective. Cyclosporine, methotrexate, and certain seizure medications also interact with it in ways that may require dose adjustments or closer monitoring.