FEED STORES across America are experiencing a surge in demand for ivermectin, a deworming drug for horses and other livestock. Prescriptions are flying off the pad for the human version, too. They are up almost 30-fold compared with 2019 according to IQVIA, a health-care analytics company. Yet it is not an epidemic of parasites that is driving sales of both kinds of ivermectin, but rather covid-19. Despite little evidence of its efficacy, it has become the pandemic quack cure du jour.

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Ivermectin’s rise parallels that of hydroxychloroquine, another much-touted “cure”. In both cases early studies showed promise before being discredited. Some statisticians have suggested that ivermectin clinical trials would not stand up to scrutiny. Many rely on small sample sizes or were not well-designed. One study was so flawed it was removed from the preprint server that hosted it over “ethical concerns”. Merck, which manufactures the drug, says there is currently “no scientific basis for a potential therapeutic effect against covid-19”.

On the other hand there is plenty of evidence it is dangerous when misused. The Food and Drug Administration (FDA), America’s regulatory agency for drugs, warns that overdosing can cause vomiting, nausea, seizures and even death. Poison control lines, set up to dispense information and handle emergencies, are swamped with calls—the American Association of Poison Control Centers received 459 in August, almost eight times as many calls about ivermectin as in the same month in 2020. In Mississippi at least 70% were about people who took the veterinary formulation, which is far more potent than the human version. Several people have been hospitalised.

Why do so many people buy into these unproven cures? Partisanship plays a big role. According to a poll that YouGov conducted on behalf of The Economist, opposition to covid-19 vaccinations is more pronounced on the right. Republicans account for 56% of respondents who say they will not get vaccinated; 25% are independents and 13% are Democrats (the remainder are “not sure” of their identity). The ideological breakdown is similar: 53% of holdouts describe themselves as conservative, and just 7% are liberal. These differences outweigh any other demographic factors. The poll also finds that 26% of Republicans (roughly the same proportion as independents) and 33% of conservatives believe that ivermectin is a “possibly” or “very” effective treatment.

How people come to believe such things is contested. One popular explanation is that influential figures persuade people to ignore medical advice. Social scientists call this a “top-down” or “elite-driven” model. Although some conservative commentators have touted ivermectin, far more have encouraged vaccination. These mixed messages suggest a hybrid “top-down, bottom-up” model, in which vaccine scepticism is also driven by peers. “Elites can (and do) try to lead public opinion,” says Christopher Wlezien, a political scientist at the University of Texas at Austin, “But they often have to follow.”

Less clear is why these beliefs are so prominent among conservatives in particular. It could be grounded in a general dislike of government intervention. One poll from Reality Check Insights, a pollster, found that Americans who think “parents, not the government” should get to decide whether their children wear face masks were likely to think the same about seatbelts. When asked whether expert risk assessment should lead governments to shut businesses, the partisan gap was similar irrespective of whether the threat was covid-19 or a hurricane.

Republicans also tend to be less educated and more religious than Democrats, which might make them trust experts less. It may also be that, rather than people’s politics determining their views on vaccination, both are determined by their pre-existing membership in other social groups. Whatever the reasons, Americans would do well to heed the words of the FDA. On August 21st it tweeted out “You are not a horse. You are not a cow. Seriously, y’all. Stop it.”