Federal health regulators are warning doctors and veterinarians against the unauthorized use of the drug ivermectin to treat Covid-19, as prescriptions of the antiparasitic drug have surged in recent weeks.

The Food and Drug Administration on Monday sent a letter to veterinarians and retailers of animal-health products urging them to help stop misuse of versions of the drug given to animals.

The Centers for Disease Control and Prevention recently issued a health advisory warning about reports of severe illness associated with misuse of the drug.

The FDA has approved ivermectin to treat some parasitic worms, as well as a topical treatment for head lice and skin conditions like rosacea. It is also used in the U.S. to treat or prevent parasites in animals.

Research hasn’t shown ivermectin treats Covid-19, and the drug isn’t authorized for that use. Yet prescriptions and other reports of its use have shot up, according to health officials, as the infectious Delta variant has spread.

Use of ivermectin against Covid-19 is dangerous, health authorities and doctors said. Calls to poison-control centers have surged as much as fivefold in recent weeks because people are taking ivermectin for Covid-19, according to federal and state officials.

Mississippi’s State Department of Health said at least 70% of its recent calls have been related to ingestion of animal formulations of ivermectin purchased at livestock supply centers. The agency didn’t give the absolute number of calls.

The Texas Poison Center Network said it got 64 calls in August reporting ivermectin exposure, more than 1.5 times the number of calls about ivermectin exposure in August as compared with the previous month, according to Texas’s Department of State Health Services. The center received 159 calls from Jan. 1 to Aug. 24 , compared with 48 in all of 2020.

The CDC warning pointed to two recent episodes where people who took ivermectin to try to prevent Covid-19 infection resulted in hospitalization.

One adult drank an injectable formulation intended for use in cattle and was hospitalized for nine days after showing up hallucinating and confused, the CDC said.

The second adult was disoriented and had difficulty answering questions or following commands after taking five ivermectin tablets over five days purchased on the internet, the CDC said.

Ivermectin was best known for treating a fly-borne disease commonly known as river blindness, before gaining attention in recent months on social media as a potential Covid-19 treatment.

The drug is generic, and costs $21 for four tablets, according to WebMDRx.

Use of the drug as a Covid-19 treatment has been promoted recently in some conservative media outlets, including Fox News, as well as by some right-wing radio hosts. Fox News didn’t immediately respond to a request for comment. Fox News parent Fox Corp. and Wall Street Journal parent News Corp share common ownership.

Phil Valentine, a Nashville-based radio host who was diagnosed with Covid-19 in July after urging people not to take the vaccine and who later died, also advocated the treatment. His family later said he regretted not having advocated more vocally for vaccines.

A group called the Front Line Covid-19 Critical Care Alliance, which supports administering ivermectin to prevent and treat Covid-19, maintains a website listing doctors known to prescribe the drug.

The doctor who founded the group prescribed ivermectin to a 51-year-old Ohio man hospitalized with Covid-19. The hospital refused, but a court intervened last week to force the hospital to give the drug, according to local media reports.

An Illinois judge on Monday sided with a hospital that refused to treat a Covid-19 patient with ivermectin. The hospital argued that the treatment is unproven and potentially unsafe, according to the State Journal-Register.

Health officials and doctors have likened the increased interest in ivermectin to the uptick in use last year of hydroxychloroquine, an antimalaria drug that has been found to have no benefit in treating Covid-19.

A recent survey of 14 studies looking at ivermectin’s use in 1,678 people as a Covid-19 treatment didn’t find evidence the drug is an effective treatment. Another 31 studies are ongoing, the researchers said.

Prescriptions for ivermectin saw a significant uptick in late 2020 and early 2021, and have surged to new levels since early July, according to the CDC.

Retail pharmacies dispensed more than 88,000 prescriptions in the week ended Aug. 13, 2021, a 24-fold increase from levels before the pandemic, according to the CDC.

Dr. Msonthi Levine, an internal medicine doctor in Beaumont, Texas, said he hadn’t received any requests for ivermectin until about a month ago; since then, about 20 patients have asked for it, both as a treatment and preventive measure.

He said he tries to talk patients out of it by pointing to established sources warning against its unauthorized use, but some people are insistent and he has prescribed it twice.

“There are some patients who feel quite strongly about it. Depending on the conversation I might give it to them just to keep them quiet,” he said.

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Most ivermectin prescriptions are being written by nurse practitioners and physician assistants, followed by family medicine and osteopathic doctors, according to Evercore ISI analyst Umer Raffat.

Some 97% of the ivermectin prescription supply is being manufactured by Edenbridge Pharmaceuticals LLC, a Parsippany, N.J.-based company.

The company didn’t immediately respond to a request for comment. It said in a statement on its website that it is aware ivermectin isn’t approved by the FDA to treat Covid-19 but is supporting certain clinical trials studying the matter.

Write to Felicia Schwartz at felicia.schwartz@wsj.com