Hydroxychloroquine has been prescribed for more than sixty years for malaria, lupus, and rheumatoid arthritis, and the way it is dosed has changed more than most patients realize. The dose a physician settles on is not chosen from a flat chart of "one size fits most." It is calculated from a patient's actual body weight, because research over the past decade has shown, with real numbers, that dosing too high for a person's size steadily raises the risk of permanent eye damage. This article explains the reasoning behind the weight-based calculation, what the underlying studies actually found, and how a typical dose is worked out in practice — so that a patient reviewing a prescription with their own doctor understands exactly what is being calculated and why.

What Hydroxychloroquine Is Actually Approved to Treat

In the United States, hydroxychloroquine is FDA-approved for the treatment and prevention of malaria caused by susceptible strains, and for the treatment of chronic discoid and systemic lupus erythematosus and rheumatoid arthritis. It is a synthetic descendant of quinine, itself drawn originally from the bark of the cinchona tree — a reminder that some of medicine's most durable tools came out of the created world long before a laboratory refined them.

It is worth stating plainly, because so many readers arrive with this question: hydroxychloroquine is not FDA-approved for the treatment or prevention of COVID-19. Several large randomized controlled trials tested it for that purpose in 2020, including the RECOVERY trial run by the University of Oxford and published in the New England Journal of Medicine, and the World Health Organization's Solidarity trial. Both found no reduction in mortality or hospital course among hospitalized patients given hydroxychloroquine compared with usual care. That is a settled finding from large, well-designed human trials, not a preliminary or in-vitro result, and it is why regulatory agencies did not approve the drug for that use. None of what follows in this article applies to that question; everything below concerns its approved uses.

The Basic Building Block: The 200 mg Tablet

Hydroxychloroquine is manufactured as a 200 mg film-coated tablet of hydroxychloroquine sulfate, which is equivalent to 155 mg of the active hydroxychloroquine base. This distinction between "salt" and "base" matters mainly to pharmacists and researchers converting between formulations, but it explains why some older malaria references quote numbers like "310 mg base" that don't obviously match the tablet strength patients hold in their hand.

For rheumatoid arthritis and lupus, typical maintenance doses fall in the range of 200 mg to 400 mg per day, usually taken once daily or split into two doses. For malaria prevention, the standard adult regimen is 400 mg once weekly, started one to two weeks before entering an area with malaria risk and continued for four weeks after leaving. For treating an acute malaria infection, a physician uses a front-loaded schedule — a larger initial dose followed by smaller doses at set intervals over the following two days. These are the numbers most references give as a starting point, but none of them is the final answer for an individual patient until body weight is factored in.

Why Weight Drives the Calculation

Hydroxychloroquine behaves differently in the body than many everyday medications. It has an enormous volume of distribution, meaning it doesn't stay in the bloodstream — it moves into tissue and stays there, sometimes for months after the last dose. It has a particular affinity for melanin-containing tissue, which includes the pigmented layer of the retina at the back of the eye. Over years of steady use, the drug can accumulate in that layer faster than the body clears it.

Because that accumulation is driven by tissue exposure over time, the relevant number isn't simply "how many milligrams a day," but "how many milligrams per kilogram of the person taking them." A 400 mg daily dose is a very different exposure for someone weighing 50 kilograms than for someone weighing 90 kilograms. Dosing by a flat milligram amount, without reference to size, was the older practice, and it is precisely what more recent research identified as the source of unnecessary long-term risk in smaller-bodied patients.

The Study That Changed the Dosing Rule

The pivotal evidence came from a retrospective cohort study led by Ronald Melles and Michael Marmor at Kaiser Permanente Northern California, published in JAMA Ophthalmology in 2014. The researchers examined more than 2,300 patients who had taken hydroxychloroquine for at least five years and screened them for retinal toxicity using modern imaging. They found the overall prevalence of retinopathy was about 7.5 percent, but the risk was anything but evenly spread. Among patients whose daily dose stayed at or below 5.0 mg per kilogram of actual body weight, the risk stayed under 2 percent even after ten years of use. Above that threshold, risk climbed sharply — reaching close to 20 percent after twenty years of continuous use in higher-dose patients. Risk also rose independently with kidney impairment, concurrent tamoxifen use, and pre-existing retinal or macular disease.

That single study reshaped clinical guidance. The American Academy of Ophthalmology used it as the basis for revised screening and dosing recommendations published in 2016, replacing an older rule of thumb that capped dosing at 6.5 mg per kilogram of ideal body weight — a formula that, in overweight or obese patients, could actually still permit doses well above what the newer, more precise, real-weight-based math would allow. The shift from "ideal weight" to "actual weight" as the denominator, paired with the lower 5.0 mg/kg ceiling, is the single biggest change in how this drug has been dosed in the last several decades, and it came directly from a large, well-conducted human cohort study rather than animal data or theory.

How a Dose Is Actually Worked Out

In practice, a physician calculates an upper daily limit by multiplying the patient's actual weight in kilograms by 5, then rounds to a practical number given that tablets only come in 200 mg strength.

Because tablets can't be split into odd fractions with any reliability, physicians often use alternating-day schedules to land close to the calculated ceiling without exceeding it. This is also why a smaller patient may be told to take less than a family member or friend with the same diagnosis — it is not a weaker treatment, it is a correctly calibrated one. For malaria prophylaxis, pediatric dosing is calculated per kilogram directly from the start, since children's doses were never standardized to a flat adult number in the first place.

Monitoring and the Physician's Role Over Time

Because retinal risk is tied to cumulative years of exposure as much as daily dose, the Academy's 2016 guidance also recommends a baseline eye examination when treatment begins, followed by annual screening with optical coherence tomography starting at the five-year mark for patients without additional risk factors — sooner for those with kidney disease, retinal disease, or very high relative dosing. Kidney function matters because the drug is partly cleared by the kidneys, and reduced clearance raises effective exposure even at an unchanged oral dose. This is the kind of detail that makes ongoing conversation with one's own physician, rather than a one-time chart lookup, the right way to manage a long-term prescription. A dose that is correct at diagnosis may need reassessment years later if weight, kidney function, or other medications change. That ongoing partnership — an informed patient and a trusted physician adjusting a real, individual treatment plan — is a far better safeguard than any general chart, including this one, could ever be.

Key takeaway: Hydroxychloroquine dosing is calculated as milligrams per kilogram of actual body weight, capped near 5 mg/kg/day, because that specific threshold — established by real cohort research — is what keeps decades of use safe for the retina.