The agency says it issued the guidance Friday because of growing interest in the drug as a COVID-19 treatment and multiple reports of patients hospitalized or needing medical support "after self-medicating with ivermectin intended for horses."
The FDA has not approved Ivermectin, which is not an antiviral, for treating or preventing COVID-19.
"Using any treatment for COVID-19 that's not approved or authorized by the FDA, unless part of a clinical trial, can cause serious harm," the FDA says.
Ivermectin tablets are FDA-approved to treat two conditions caused by parasitic worms: intestinal strongyloidiasis (a chronic infection) and onchocerciasis (river blindness). Ivermectin are approved as a topical treatment for head lice and skin conditions such as rosacea.
Some forms of ivermectin are used to prevent heartworm disease in animals, as well as certain internal and external parasites.
The FDA says, "It's important to note that these products are different from the ones for people, and safe when used as prescribed for animals, only."
The guidance points out that the concentrations of ivermectin for cows and horses can be highly toxic to humans.
"If you have a prescription for ivermectin for an FDA-approved use, get it from a legitimate source and take it exactly as prescribed," the guidance says. "Taking large doses of this drug is dangerous and can cause serious harm."
Interactions with other drugs, such as blood thinners, are also potentially dangerous even at the levels specified in approved uses, the FDA says.
The FDA has not reviewed data to support use of ivermectin to treat or prevent COVID-19, but research is beginning.
An article published on Thursday in JAMA found that ivermectin, tested in a randomized trial of 476 patients, did not significantly shorten duration of symptoms for adults with mild COVID-19 who received a 5-day course of ivermectin compared with placebo.
As for adverse effects, the most commonly reported in the JAMA study was headache, reported by 104 patients (52%) in the ivermectin group and 111 (56%) in the placebo group. The most common serious adverse event was multiorgan failure, which occurred in four patients (two in each group).
"The findings do not support the use of ivermectin for treatment of mild COVID-19, although larger trials may be needed to understand the effects of ivermectin on other clinically relevant outcomes," the authors write.
Excitement about the drug has grown after some smaller studies have shown positive results for the drug related to COVID-19.
However, the National Institutes of Health says, "[M]ost of these studies had incomplete information and significant methodological limitations."
The NIH's COVID-19 Treatment Guidelines, in guidance last updated Feb. 11, said there is insufficient evidence to recommend either for or against the use of ivermectin for the treatment of COVID-19.
That recommendation followed guidance in August that recommend against ivermectin's use in treating or preventing COVID-19.
"Results from adequately powered, well-designed, and well-conducted clinical trials are needed to provide more specific, evidence-based guidance on the role of ivermectin in the treatment of COVID-19," the NIH panel writes.