Hydroxychloroquine is one of the few medicines in routine use today whose story reaches back through synthetic chemistry all the way to a tree bark gathered in the forests of Peru. This article traces that two-century line from cinchona bark to quinine to chloroquine to hydroxychloroquine, explains how the drug actually works in the body, and lays out plainly what strong evidence supports for its established uses, what it does not support, and why careful monitoring matters for anyone taking it long term.

From the Andes to the Apothecary

For centuries, indigenous peoples of the Andes and, later, Spanish colonists and Jesuit missionaries in seventeenth-century Peru recognized that the ground bark of certain cinchona trees relieved the shaking fevers of malaria. Europeans came to call it "Jesuit's bark" or "Peruvian bark," and it was carried back across the Atlantic as one of the first effective plant-derived treatments for a specific disease rather than a general tonic. It is worth pausing on that fact: a molecule tucked into the bark of a tree, growing wild on a mountainside, turned out to interrupt the life cycle of a parasitic organism invisible to the naked eye. That kind of fit between a natural compound and a specific disease process is a recurring pattern in pharmacology, and one that rewards humility about how much order is built into the created world before any chemist ever isolates it.

The active principle remained a mystery until 1820, when the French chemists Pierre-Joseph Pelletier and Joseph Bienaimé Caventou isolated quinine from cinchona bark in Paris. This was a landmark moment in the young science of pharmacology: for the first time, physicians could dose a purified alkaloid rather than a variable quantity of crushed bark. Quinine became the standard treatment for malaria worldwide for more than a century, though supply depended almost entirely on cinchona plantations, first wild-harvested in South America and later cultivated under Dutch control in Java.

Synthetic Chemistry Answers a Wartime Need

That dependence on a single overseas source became a strategic vulnerability, and it drove much of the twentieth-century push to synthesize antimalarial compounds from scratch. German chemist Hans Andersag, working at Bayer, synthesized chloroquine in 1934 and gave it the trade name Resochin. German researchers at the time judged it too toxic for human use and set it aside in favor of related compounds. When the Second World War cut off Allied access to cinchona-growing regions in the Pacific, the United States government funded a large antimalarial drug development program, and American researchers revisited chloroquine, finding it both effective and, at proper doses, tolerable.

Hydroxychloroquine followed directly from that effort. In 1946, chemists Alexander R. Surrey and Henry F. Hammer, working at Sterling-Winthrop Research Institute in the United States, modified the chloroquine molecule by adding a hydroxyl group, producing a compound with a meaningfully better safety margin while retaining antimalarial activity. The U.S. Food and Drug Administration approved hydroxychloroquine in 1955, sold under the brand name Plaquenil, and it gradually displaced chloroquine in many clinical settings precisely because it caused fewer side effects at therapeutic doses.

How Hydroxychloroquine Works in the Body

Hydroxychloroquine is a weak base that concentrates inside acidic cellular compartments, particularly lysosomes and endosomes, and raises their internal pH. In the malaria parasite, this disrupts the way the organism detoxifies heme released as it digests hemoglobin, which is lethal to the parasite. That mechanism explains its antimalarial effect, though widespread parasite resistance has since limited chloroquine-class drugs' usefulness against many malaria strains in much of the world.

In the human immune system, the same pH-raising action has a different but equally important consequence. It interferes with antigen processing and presentation by immune cells and dampens signaling through certain toll-like receptors, particularly TLR7 and TLR9, which sense nucleic acids and help drive autoimmune inflammation. The practical result is a modest but real reduction in the production of inflammatory cytokines. This is why a drug developed to kill a parasite turned out, decades later, to be genuinely useful for diseases where the immune system attacks the body's own tissue.

Established, Evidence-Based Uses Today

Hydroxychloroquine's FDA-approved indications reflect this dual history:

These are not marginal or preliminary findings. The evidence supporting hydroxychloroquine in lupus and rheumatoid arthritis comes from decades of accumulated randomized trials and large observational cohorts, and its place in standard rheumatology treatment guidelines reflects that maturity of evidence.

Safety and Monitoring: The Retinal Question

The most clinically significant long-term risk of hydroxychloroquine is retinal toxicity, a slowly developing injury to the light-sensing cells of the retina that, in advanced cases, produces a distinctive "bull's-eye" pattern of damage and can cause irreversible vision loss if not caught early. Risk rises with cumulative dose and duration of use, becoming more notable after five years of continuous therapy, and is higher in patients on higher weight-based doses, those with kidney disease, and those already taking other retina-affecting medications.

The American Academy of Ophthalmology revised its screening recommendations in 2016 specifically because more sensitive imaging techniques revealed toxicity could occur at doses once considered universally safe. Current guidance calls for a baseline eye examination before starting the drug, dosing based on real body weight and generally kept at or below about five milligrams per kilogram per day, and annual screening with optical coherence tomography and visual field testing beginning after five years of use, sooner in higher-risk patients. This is a case where personal responsibility genuinely matters: patients who keep their scheduled eye exams and report visual changes promptly can catch toxicity at a stage where stopping the drug prevents further damage. It is a partnership between patient and physician, not a task either can safely skip.

The COVID-19 Chapter: What the Evidence Actually Showed

In early 2020, laboratory studies showed hydroxychloroquine could inhibit the virus that causes COVID-19 in cell culture, and a small, non-randomized study from Didier Raoult's team at the IHU Méditerranée Infection in Marseille, France, published in the International Journal of Antimicrobial Agents in March 2020, reported reduced viral detection in roughly three dozen patients given hydroxychloroquine, with and without azithromycin. That study drew wide public attention, but it was also widely criticized on methodological grounds, including its small size, lack of randomization, and the exclusion of some patients from the final analysis.

The question was then tested properly. The RECOVERY trial, a large randomized controlled trial run by the University of Oxford involving thousands of hospitalized COVID-19 patients in the United Kingdom, found no meaningful difference in 28-day mortality between patients given hydroxychloroquine and those receiving usual care, with mortality running close to a quarter of patients in both groups; the hydroxychloroquine arm was stopped in June 2020 once that result was clear. The World Health Organization's Solidarity trial, conducted across many countries, reached a similar conclusion and also discontinued its hydroxychloroquine arm. On the strength of this randomized evidence, the U.S. FDA revoked its emergency use authorization for hydroxychloroquine in hospitalized COVID-19 patients in June 2020, citing both the absence of demonstrated benefit and concerns about heart rhythm effects, particularly QT interval prolongation, in acutely ill patients.

That sequence is worth stating plainly rather than politically: early cell-culture and small clinical signals suggested a possible benefit, and when tested in large, well-designed randomized trials, that benefit did not materialize for hospitalized COVID-19 patients. Hydroxychloroquine is not approved for COVID-19 treatment or prevention. None of this diminishes its established, well-evidenced role in malaria, lupus, and rheumatoid arthritis, and patients with questions about any off-label use of a long-approved medicine retain the right to discuss the full evidence, including its limits, candidly with their own physician.

Key takeaway: Hydroxychloroquine's two-century path from cinchona bark to modern pharmacy is a genuine medical success story for malaria, lupus, and rheumatoid arthritis, provided it is used at correct doses with the retinal monitoring good evidence recommends, and its COVID-19 chapter is a case study in how large randomized trials, not small early signals, should settle a clinical question.