Here we go again.

The latest drug being touted on social media as a panacea for the pandemic is a veterinary medicine used to treat parasites in farm animals. That formulation of ivermectin is so strong that it can be highly toxic to humans.

Which is why the Food and Drug Administration tweeted this week: “You are not a horse. You are not a cow. Seriously, y’all. Stop it.”

Yet people clearly are not stopping. The Centers for Disease Control and Prevention reported Thursday that poison control centers across the country have had a fivefold increase in calls about “human exposure” to ivermectin, although the agency did not specify the actual number of calls.

The ivermectin brouhaha has echoes of the politicized debate over hydroxychloroquine, the now-disproven antimalarial that then-President Donald Trump championed as a COVID-19 treatment. But there is a big difference. Ivermectin is being cast by vaccine opponents as an alternative to vaccines, which didn’t exist when hydroxychloroquine was all the rage.

Ivermectin is just one of hundreds of existing drugs that have been screened in labs or in small human studies to see whether they might work against the coronavirus. Here is the scoop on ivermectin and other compounds capturing the fancy of researchers, folks on social media, and anti-vaccine groups.

Ivermectin

Ivermectin does have approved human uses in the United States. A topical form is sometimes prescribed to treat head lice and skin disorders such as rosacea. A low-dose tablet form is effective against certain intestinal parasites.

Why might it work against COVID-19? In studies of human cells in lab dishes, ivermectin has been shown to reduce viral RNA, the genetic material that the coronavirus uses to make copies of itself inside cells. Researchers theorize that ivermectin blocks a transport molecule that the virus needs to get into the cell’s control center, or nucleus.

But in real people, research data on ivermectin’s effectiveness against COVID-19 are still scarce, conflicting, and in one prominent study, fraudulent. Some experts believe that to work, ivermectin would have to be given in dangerously high doses.

And that’s where the farm-animal formulation comes in. It is a highly concentrated paste, often used for deworming large animals. It can be easily purchased at feed stores and does not require a prescription. In humans, this formulation can cause a rash, nausea, vomiting, abdominal pain, neurologic disorders, and potentially severe liver damage, according to an alert from the Mississippi State Department of Health.

The alert was issued because that state, which has the second-lowest rate of vaccinations in the United States, had a spike in calls to poison control centers from people who consumed the ivermectin paste. Most callers had mild symptoms, but one had to be hospitalized.

“You wouldn’t get your chemotherapy at a feed store,” Mississippi State Health Officer Thomas Dobbs said in a Zoom call last week. “I mean, you wouldn’t want to treat your pneumonia with your animal’s medication. It can be dangerous to get the wrong doses of medication, especially for something that’s meant for a horse or a cow.”

Retail pharmacies also are seeing an increase in prescriptions for ivermectin, the CDC reported Thursday. A recent study found scripts jumped from 3,600 per week before the pandemic to a peak of 39,000 in January.

Despite that troubling trend, it’s easy to find ivermectin testimonials on social media. “Less than a hour after taking Ivermectin paste per my body weight I was mostly symptom free. ... Was in bad shape until then!” claimed one user.

Daniel Horowitz, a conservative writer and podcaster, tweeted that ivermectin is a “miracle drug” and that warnings against it are “blood libel.”

Fenofibrate

Fenofibrate, brand name TriCor, is a low-cost generic drug approved to treat high cholesterol.

The idea that it might work against COVID-19 has been catching on in scientific and social media circles, based mostly on studies of coronavirus-infected cells in lab dishes. An Israeli research group also recently reported an intriguing but tiny study of 15 hospitalized COVID-19 patients receiving oxygen. A week after being put on fenofibrate, 14 of them discontinued the oxygen support.

Three large, rigorous clinical trials of fenofibrate are underway around the world — one of them led by University of Pennsylvania physician-scientist Julio A. Chirinos.

In infected cells in lab dishes, fenofibrate “markedly inhibited viral replication,” explains the summary of Chirinos’ study, which aims to enroll 700 patients. “Recent studies suggest that COVID-19 progression is dependent on metabolic mechanisms” involving blood fats. “Fenofibrate also has immunomodulatory effects that may be beneficial in the setting of COVID.”

Fluvoxamine

Fluvoxamine is in a class of drugs, called selective serotonin reuptake inhibitors (SSRI), that act on certain chemical messengers in the brain. Fluvoxamine is approved to treat obsessive-compulsive disorder and depression.

How could an OCD drug fight an infection?

Again, the evidence and theories are pretty sketchy. Fluvoxamine has anti-inflammatory properties in cells in lab dishes, so in theory it could help tamp down the runaway inflammation that occurs in severe COVID-19.

But a National Institutes of Health expert panel says it’s not clear whether the effects seen in lab dishes “also occur in human beings and are clinically relevant in the setting of COVID-19.”

“There is insufficient evidence for the COVID-19 Treatment Guidelines Panel to recommend either for or against the use of fluvoxamine,” the panel concluded.

The unceasing appeal of unproven drugs to those with reluctance to get vaccinated prompted George Takei — a.k.a. Mr. Sulu on Star Trek — to tweet: “No bleach. No hydroxy. No ivermectin. Just stop. Please. Get the vaccine.”