Ahead of full US authorisation of the Pfizer coronavirus vaccine, the federal Food and Drug Administration (FDA) had a simple message for Americans contemplating using ivermectin, a medicine used to deworm livestock, instead of getting a Covid shot.

Mississippi state health officials issued a recent alert prompted by increasing calls of people with potential ivermectin exposure taken to prevent or treat COVID-19 infection. While the drug has FDA approval for certain uses in animals and humans, over 70% of the recent calls involved livestock formulations, officials said.

The FDA previously issued concerns over self-medication with ivermectin intended for animals, noting some people might mistakenly believe the drug can substitute for ivermectin intended for humans.

"Animal drugs are often highly concentrated because they are used for large animals like horses and cows, which can weigh a lot more than we do—a ton or more," reads an agency webpage. "Such high doses can be highly toxic in humans."

Ivermectin toxicity can result in symptoms like "rash, nausea, vomiting, abdominal pain, neurologic disorders, and potentially severe hepatitis requiring hospitalization," Mississippi health officials noted. An overdose on ivermectin can result in seizures, coma and even death, the FDA warns.

In comments shared widely on social media, Dr Pierre Kory, a pulmonary and critical care specialist at Aurora St Luke’s medical centre in Milwaukee, called ivermectin a “wonder drug”.

Experts said then that test results suggesting ivermectin could inhibit replication of the SARS-CoV-2 virus did not amount to official authorisation for use.

“It is a far cry from an in-vitro lab replication to helping humans,” Dr Nasia Safdar, medical director of infection prevention at the University of Wisconsin-Madison Hospital, told the Associated Press.

Eight months later, many US states are struggling to boost vaccination rates and contain the contagious Delta variant, some running out of intensive care capacity.

The vast majority of hospitalisations and deaths in the US involve unvaccinated people. On Saturday night in Alabama, the state with the lowest vaccination rate, Trump told a crowd to get the shot. He was booed and jeered.

On Sunday the US surgeon general, Dr Vivek Murthy, told CNN: “The best protection we have against Covid-19 is the vaccine, and if you get Covid-19, we do have treatments that work.

“Ivermectin is not one of them.”

The Pfizer shot was formally approved on Monday.

The FDA accompanied its Saturday tweet with a fact sheet. In answer to the question “Should I take ivermectin to treat Covid-19”, it said: “No. While there are approved uses for ivermectin in people and animals, it is not approved for the prevention or treatment of Covid-19.

“You should not take any medication to treat or prevent Covid-19 unless it has been prescribed to you by your healthcare provider and acquired from a legitimate source.”

Additional studies, it said, were needed to determine whether ivermectin “might be appropriate to prevent or treat coronavirus or Covid-19”.

The FDA also said some animal owners may be having difficulties finding ivermectin because of humans seeking Covid cures.

Ivermectin tablets are approved for use in humans, the FDA said, for “the treatment of some parasitic worms”. Formulations can be used by prescription only for the treatment of “skin conditions such as rosacea” and “external parasites such as headlice” – or nits.