Hydroxychloroquine is often described as the drug that replaced chloroquine once malaria parasites became resistant. That story is only half true, and the accurate version is more interesting. This article traces the real lineage of both drugs, from a Peruvian tree bark to a German laboratory to the molecular mechanism that let a single-celled parasite outwit twentieth-century medicine, and explains plainly what hydroxychloroquine can and cannot do that chloroquine cannot.

From Jesuit's Bark to a Pure Molecule

For centuries, indigenous peoples of the Andes used the bark of the cinchona tree to treat fevers. Jesuit missionaries carried this knowledge to Europe in the seventeenth century, where the remedy became known as "Jesuit's powder" or "Jesuit's bark." It worked, though no one understood why, because the bark contains quinine, an alkaloid that interferes with how the malaria parasite disposes of toxic byproducts as it feeds on red blood cells. In 1820, French chemists Pierre-Joseph Pelletier and Joseph Bienaimé Caventou isolated pure quinine from the bark, turning a folk remedy into a defined pharmaceutical compound. It is worth pausing on this: a compound capable of treating one of humanity's oldest and deadliest diseases was quietly present in the bark of a tree long before anyone had the chemistry to find it. That kind of provision, embedded in the created world and waiting on human discovery, recurs throughout the history of medicine.

Quinine remained the primary antimalarial for more than a century, but it had real drawbacks: unpleasant side effects known as cinchonism (ringing in the ears, nausea, visual disturbance), and a supply chain dependent almost entirely on cinchona plantations that the Dutch controlled in Java. When Japan occupied Java in 1942, Allied forces lost access to roughly ninety percent of the world's quinine supply, and the search for a synthetic substitute became a wartime priority.

The German Synthesis and Chloroquine's Wartime Rise

The synthetic path to chloroquine actually predates the war. German chemist Hans Andersag, working at Bayer, synthesized the compound in 1934 and named it Resochin. German researchers at the time judged it too toxic for clinical use and set it aside in favor of a related compound. When American and Allied researchers, working under the U.S. Board for the Coordination of Malarial Studies, later reexamined captured German research and their own synthesized analogs during the North African and Pacific campaigns, they found Resochin was in fact effective and reasonably well tolerated at proper doses. Renamed chloroquine, it was approved for use in the United States in 1949 and quickly became the backbone of malaria treatment and prevention worldwide. It was inexpensive to manufacture, effective against all four human malaria species then recognized, and simple enough to administer that it anchored the World Health Organization's Global Malaria Eradication Programme launched in 1955.

Resistance Emerges: The Parasite Fights Back

Chloroquine's dominance did not last uncontested. Clinical reports of chloroquine treatment failures in Plasmodium falciparum surfaced independently in the late 1950s along the Thailand-Cambodia border and in Colombia. Resistant parasites spread through Southeast Asia and South America over the following two decades, and by 1978 chloroquine-resistant falciparum malaria had been documented in East Africa, from where it spread across much of sub-Saharan Africa through the 1980s. This mattered enormously, because Africa carries the overwhelming majority of the world's malaria burden, and children under five bear the largest share of malaria deaths.

The molecular reason for this resistance was worked out through laboratory research in the 1990s. A landmark study led by David Fidock, working with researchers including Thomas Wellems at the U.S. National Institutes of Health, and published in the journal Molecular Cell in 2000, identified mutations in a gene called pfcrt, which codes for a transporter protein on the membrane of the parasite's digestive vacuole. In sensitive parasites, chloroquine accumulates inside this vacuole and blocks the parasite from safely disposing of heme, a toxic byproduct of digesting hemoglobin, which kills the parasite. The mutated pfcrt transporter effectively pumps chloroquine back out of the vacuole before it can do its job. This was laboratory and genetic research, not a hypothesis, and it explained why resistance appeared, spread geographically through parasite migration and human travel, and proved so durable once established.

Hydroxychloroquine: A Safer Cousin, Not a Resistance Fix

Hydroxychloroquine was not developed in response to resistance at all. It was synthesized in 1946 by Alexander Surrey and Henry Hammer at Sterling-Winthrop Research Institute, who modified chloroquine's structure by adding a hydroxyl group to one of its side chains. The goal was tolerability, not novel efficacy. It was approved by the FDA in 1955 under the brand name Plaquenil and was found to retain essentially the same antimalarial potency as chloroquine while causing less gastrointestinal upset and, at equivalent long-term dosing, a somewhat more favorable margin against retinal toxicity, though hydroxychloroquine can still cause retinopathy with prolonged high-dose use, which is why ophthalmologists recommend regular eye monitoring for patients on long-term therapy.

Here is the point often missed in popular accounts: because hydroxychloroquine and chloroquine are both 4-aminoquinolines that act through the same mechanism inside the same digestive vacuole, the pfcrt mutations that confer chloroquine resistance also confer cross-resistance to hydroxychloroquine. Hydroxychloroquine is not a rescue drug for chloroquine-resistant malaria. Where P. falciparum resistance is established, hydroxychloroquine does not reliably work either. Its rise to prominence had much more to do with rheumatology than with tropical medicine: physicians treating systemic lupus erythematosus and rheumatoid arthritis found it useful for its immunomodulatory effects, and its better long-term tolerability made it the preferred 4-aminoquinoline for chronic, years-long use in these autoimmune conditions, a role chloroquine never held as comfortably.

To be precise about what the evidence supports:

Where Malaria Treatment Stands Today

Because both drugs lost reliability against resistant P. falciparum, global malaria treatment guidelines moved on. The World Health Organization recommended artemisinin-based combination therapies, or ACTs, as first-line treatment for falciparum malaria starting in the early 2000s. Artemisinin itself has its own remarkable origin story: it was isolated from the sweet wormwood plant, Artemisia annua, by Chinese scientist Tu Youyou in 1972 during a government research program, work for which she received the Nobel Prize in Physiology or Medicine in 2015. ACTs pair a fast-acting artemisinin derivative with a longer-acting partner drug, reducing the chance that surviving parasites develop further resistance.

Chloroquine and hydroxychloroquine have not disappeared from malaria medicine entirely. They remain reasonable options for treating P. vivax, P. ovale, and P. malariae infections in most regions, and for both treatment and prevention in the shrinking number of areas where P. falciparum has not developed resistance. Travelers should never assume this without checking current regional resistance data with their physician or a travel medicine clinic, since resistance patterns are geographically specific and continue to shift, including documented chloroquine-resistant P. vivax in parts of Indonesia and Papua New Guinea since the late 1980s.

It is also worth noting, briefly and factually, that hydroxychloroquine received an FDA Emergency Use Authorization for hospitalized COVID-19 patients in March 2020, which was revoked that June after large randomized controlled trials, including the RECOVERY trial run by Oxford University and published in 2020, found no mortality benefit. Hydroxychloroquine is not an FDA-approved treatment for COVID-19. This history is a reminder of a broader principle worth keeping in mind: a drug's long track record in one disease does not automatically transfer to another, and that judgment belongs to careful trial evidence, not assumption in either direction.

What This History Should Teach Patients

The chloroquine story is a case study in how living organisms adapt under selective pressure, and how medicine must adapt in turn. It is also a reminder that good stewardship of the body means working with a physician who knows the current resistance patterns in a given region, the actual approved uses of a given drug, and a patient's individual health history, rather than relying on drug names alone. Hydroxychloroquine remains a genuinely valuable medicine: safe and effective for chloroquine-sensitive malaria, and a mainstay, well-supported by decades of rheumatologic evidence, for lupus and rheumatoid arthritis. Understanding what it is for, and equally what it is not for, is part of the informed consent every patient deserves before starting any medication.