Ivermectin is made from a natural compound produced by a soil bacterium called Streptomyces avermitilis. The bacterium was first isolated from a soil sample collected in the woods near a golf course in Japan in the 1970s. Scientists then chemically modified the bacterium’s natural product to create ivermectin, making it a semi-synthetic drug: part natural, part lab-engineered.
The Soil Bacterium Behind Ivermectin
The story starts with Satoshi Ōmura, a Japanese microbiologist at Kitasato University in Tokyo. Ōmura spent years collecting soil samples from across Japan and culturing the microorganisms he found in them. Among the thousands of strains he isolated, one stood out: Streptomyces avermitilis, a bacterium that naturally produces a family of compounds called avermectins. These compounds turned out to be remarkably potent at killing parasites.
In 1972, the pharmaceutical company Merck established a collaboration with Ōmura’s lab. William Campbell, a parasitologist at Merck’s research division, tested the avermectins against parasitic worms in animals and found striking results. Campbell and his colleagues then chemically modified avermectin to create ivermectin, a version that was even more effective against parasitic infections. Both Ōmura and Campbell shared the 2015 Nobel Prize in Physiology or Medicine for this work.
How Avermectin Becomes Ivermectin
Ivermectin isn’t pulled directly from a bacterium and put into a pill. The bacterium produces avermectin through fermentation, similar to how yeast produces alcohol during brewing. Scientists grow large batches of Streptomyces avermitilis under controlled conditions, and the bacteria release avermectin into the surrounding liquid.
That raw avermectin then undergoes a chemical modification in the lab. The key step is a process called selective hydrogenation, where chemists add hydrogen atoms to a specific spot on the avermectin molecule (a double bond between carbon atoms 22 and 23). This is done using a palladium catalyst and a borohydride reducing agent under carefully controlled temperatures. The result is 22,23-dihydroavermectin B1, which is ivermectin’s technical name. Pharmaceutical-grade ivermectin contains at least 80% of one form of this molecule (B1a) and less than 20% of a closely related form (B1b).
This combination of biological fermentation and chemical modification is why ivermectin is classified as semi-synthetic. The bacterium does the heavy lifting of building the complex molecular scaffold, and chemists fine-tune it.
How It Kills Parasites
Ivermectin targets a type of nerve channel found in invertebrates, specifically glutamate-gated chloride channels. These channels control nerve signaling in parasitic worms and insects. Ivermectin binds to these channels and forces them open, which floods the parasite’s nerve and muscle cells with chloride ions. This essentially short-circuits the parasite’s nervous system, causing paralysis and death.
The reason ivermectin is safe for humans and other mammals at prescribed doses is that our nervous systems don’t rely on the same type of chloride channel. We have them in the brain, but ivermectin doesn’t cross the blood-brain barrier easily in healthy individuals, so it targets the parasite while largely leaving the host alone.
What It’s Used For
Ivermectin has been on the World Health Organization’s List of Essential Medicines since 1987, originally for river blindness (onchocerciasis), a parasitic disease spread by black flies that can cause permanent vision loss. Its approved uses have expanded significantly since then. The WHO now lists it as essential for treating lymphatic filariasis (added in 2011), several intestinal worm infections including roundworm, whipworm, and hookworm (added in 2017), and scabies (added in 2019).
The path from soil bacterium to human medicine took about a decade. Campbell proposed testing the avermectin family in humans in 1977, after seeing strong results against dog heartworm and other animal parasites. The first successful human trial took place in 1981-1982, led by Mohammed Aziz at Merck, focusing on river blindness patients.
What’s in an Ivermectin Tablet
The active ingredient in a standard ivermectin tablet (sold under the brand name Stromectol) is 3 mg of ivermectin. The rest of the tablet is made of common pharmaceutical fillers and binders: microcrystalline cellulose (a plant fiber that gives the tablet its structure), pregelatinized starch (helps it dissolve), magnesium stearate (prevents ingredients from sticking to manufacturing equipment), butylated hydroxyanisole (a preservative), and citric acid. None of these inactive ingredients are unusual; they appear in many common medications.