Ivermectin is one of the most consequential medicines of the past half-century, yet its story begins with a handful of dirt. This article explains how a Japanese microbiologist and an American parasitologist turned a soil bacterium into a family of antiparasitic drugs, how one of those drugs became the backbone of the campaign against river blindness, and why that work earned the 2015 Nobel Prize in Physiology or Medicine. It also sets out plainly what ivermectin is and is not approved to treat today, so readers can discuss it with their own physician on solid ground.

A soil sample from Shizuoka

In the early 1970s, Satoshi Ōmura, a chemist at the Kitasato Institute in Tokyo, was running a systematic search for useful compounds made by soil microbes. His team collected samples, isolated bacteria, grew them in culture and screened the products. Much of modern medicine had come from the same approach: the streptomycete bacteria that live in ordinary soil had already yielded streptomycin, tetracyclines and other antibiotics.

One sample, collected near a golf course at Kawana in Itō, on the coast of Shizuoka Prefecture, contained a streptomycete strain that Ōmura's group cultured and characterised. Under a research agreement signed in 1973, Kitasato sent promising cultures to Merck Sharp & Dohme Research Laboratories in New Jersey for biological testing. Ōmura's lab supplied the microbes and the chemistry; Merck supplied large-scale animal screening.

A remarkable detail, often noted by historians of the discovery, is that later searches of soils elsewhere did not turn up another organism producing these compounds in the same way. Whether one calls it serendipity or providence, it is worth pausing on: a molecule that would protect hundreds of millions of people came from a single microbe in a single patch of ground, waiting in the created order for someone patient enough to look.

From fermentation broth to drug: William Campbell's contribution

At Merck, William C. Campbell, an Irish-born parasitologist, led the team testing culture broths against parasitic worms. Around 1975, broth from the Kitasato strain was found to clear a roundworm infection (Nematospiroides dubius, now called Heligmosomoides polygyrus) in laboratory mice. The active ingredients were purified and named avermectins, and the organism was named Streptomyces avermitilis. The key findings were published in 1979 in Antimicrobial Agents and Chemotherapy.

Merck chemists then modified the natural compound. Adding two hydrogen atoms to avermectin B1 produced ivermectin, which proved both more effective and better tolerated. It reached the veterinary market in 1981 for livestock and later for dogs, and quickly became one of the best-selling animal health products in the world.

Its mechanism explains both its power and its safety in people:

River blindness and the first human trials

Onchocerciasis, or river blindness, is caused by the worm Onchocerca volvulus, transmitted by blackflies that breed in fast-flowing rivers. Adult worms live in nodules under the skin and release millions of tiny larvae called microfilariae, which migrate through skin and eyes. The result is relentless itching, disfiguring skin disease and, over years, blindness. More than 99 percent of cases are in sub-Saharan Africa, with smaller foci in Yemen and Latin America. In heavily affected villages, fertile river valleys were abandoned and it was common for a large share of older adults to be blind, often led by children holding a stick.

Before ivermectin, treatment options were poor. Diethylcarbamazine killed microfilariae but could trigger severe inflammatory reactions, and suramin, which kills adult worms, was toxic. The West African Onchocerciasis Control Programme, launched in 1974, relied mainly on spraying rivers to kill blackfly larvae.

Campbell and colleagues recognised that ivermectin might work against Onchocerca. Merck physician Mohammed Aziz led early human studies in Senegal, and a 1982 report in The Lancet showed that a single oral dose sharply reduced microfilariae in the skin with far milder reactions than older drugs. Later studies confirmed that one dose suppresses skin microfilariae for months and temporarily halts reproduction by adult female worms. Ivermectin does not reliably kill adult worms, which can live more than a decade, so it must be given repeatedly, typically once or twice a year. This is the honest limitation behind decades of mass treatment.

"As much as needed, for as long as needed": the Mectizan Donation Program

Ivermectin for human use, under the name Mectizan, was registered in France in 1987. The people who needed it most could not pay, and no government or agency had committed to buying it. In October 1987, Merck, under chief executive Roy Vagelos, announced it would donate the drug free of charge to treat river blindness, in whatever quantity was needed, for as long as needed. It was the first open-ended drug donation of its kind.

The Mectizan Donation Program, administered from the Task Force for Global Health in Decatur, Georgia, with an independent expert committee reviewing applications, coordinated distribution with ministries of health, the World Health Organization and non-governmental partners. In 1998 the donation was extended to lymphatic filariasis in African countries where it overlaps with onchocerciasis, given together with albendazole donated by another manufacturer. Over the decades the program has supplied billions of treatments.

Perhaps its most important innovation was practical rather than pharmacological. Through the African Programme for Onchocerciasis Control (1995 to 2015), the model of community-directed treatment with ivermectin put responsibility in the hands of villages themselves. Communities chose their own distributors, decided when and how to hand out tablets, and kept their own records. It worked because it respected local people as capable stewards of their own health rather than passive recipients, and the same volunteer networks were later used for vitamin A and other interventions.

The results are measurable:

Challenges remain. In parts of Central Africa, people heavily infected with another worm, Loa loa, can develop rare but serious brain inflammation after ivermectin, which has required "test and not treat" strategies using rapid blood counts. Mass campaigns have generally excluded pregnant women and very small children. Researchers also watch for signs of reduced parasite response, though widespread resistance in human onchocerciasis has not been established.

The 2015 Nobel Prize

In October 2015, the Nobel Assembly at the Karolinska Institute awarded half of the Prize in Physiology or Medicine jointly to William C. Campbell and Satoshi Ōmura "for their discoveries concerning a novel therapy against infections caused by roundworm parasites." The other half went to Tu Youyou of China for discovering artemisinin, an antimalarial drawn from sweet wormwood. The pairing was apt: both discoveries came from natural products, and both chiefly benefited the world's poorest people.

Ōmura has often described the work as a partnership between Japanese academic microbiology and an American company's development capacity. Campbell has spoken of a team effort spanning chemists, parasitologists and clinicians. Both men consistently credited many colleagues, a reminder that great medicines are rarely the work of one person.

What ivermectin is and is not approved for today

Readers will likely have encountered ivermectin in recent debates, so precision matters. In the United States, oral ivermectin is FDA-approved for strongyloidiasis of the intestine and onchocerciasis. Topical formulations are approved for head lice and for the inflammatory lesions of rosacea. Oral ivermectin is widely used for scabies, including crusted scabies, and is approved for it in some countries, but in the United States that use is off-label. Ivermectin is also on the WHO Model List of Essential Medicines.

During the COVID-19 pandemic, a 2020 laboratory study from Monash University, published in Antiviral Research, found that ivermectin suppressed SARS-CoV-2 in cell culture. The concentrations used, however, were far higher than those reached in human blood at approved doses. Subsequent large randomised trials in humans, including TOGETHER (New England Journal of Medicine, 2022), ACTIV-6 (JAMA, 2022 and 2023) and the UK's PRINCIPLE trial, did not find that ivermectin reduced hospitalisation or death; PRINCIPLE reported a modest difference in self-reported recovery time without effect on serious outcomes. Ivermectin is not approved for treating or preventing COVID-19. Informed consent means having the full picture, and that picture includes these results.

Like any medicine, ivermectin should be taken only in human formulations, at doses set by a clinician who knows your history, other medications and travel exposures. Veterinary products are concentrated for large animals, and poisonings have occurred when people used them.

Key takeaway: Ivermectin, born from a single Japanese soil microbe and a fruitful partnership between two scientists, has protected hundreds of millions of people from river blindness through a donation program that trusted communities to care for their own, and its proven uses deserve to be understood accurately by every patient and physician.