Just to be clear:
Ivermectin was originally developed as a veterinary medicine, and was first marketed as such in 1981. It was fully approved for use in humans in 1988, has full FDA approval, and is on the WHO’s List of Essential Medicines. It’s been used by hundreds of millions of people since 1988. When, as they say, it’s used as directed, for its intended purpose, it’s a safe and effective medication. Its intended purpose is as an antiparasitic, and more specifically a dewormer.
Given the typical House plot structure, I’m assuming someone suggested the patient’s symptoms might be caused by parasitic worms, and suggested their usual diagnosis-by-treatment: if they administer ivermectin (a common and effective dewormer) and the patient improves, the problem is worms, and they continue treatment for that; if not, it’s something else, and they move on.
The issue with COVID is that for somewhat obscure reasons a lot of people have latched onto ivermectin as a supposed treatment. Ivermectin is an antiparastic, and COVID is of course caused by a virus, so ivermectin is (almost certainly) useless. Prescription ivermectin is quite safe, but quite pointless for treating COVID.
It gets worse than that, though. Ivermectin is a common prescription dewormer in humans, but in the U.S., parasitic worms in humans aren’t particularly common, and you have to actually get a qualified medical professional to give you a prescription to obtain it, which makes it pretty hard to get for those who think it treats COVID. On the other hand, parasitic worms are fairly common in livestock, and veterinary formulations of ivermectin are widely available. So, a lot of people have tried self-medicating with veterinary ivermectin.
The dosages and concentrations intended for horses (maybe the most widely available formulation in the U.S.) are, of course, wildly wrong for application in humans. But it gets worse. Veterinary formulations included ingredients that can be highly toxic to humans. Prescription ivermectin is safe but useless for treating COVID. Veterinary ivermectin is not only useless for treating COVID, it’s actively dangerous.
(This has, by the way, led to quite a few people, including on this board, to try to fight misinformation about ivermectin by constantly referring to it as “horse medicine”, even when discussing prescription ivermectin formulated for humans).
ETA: and ninja’d by a reference to my own previous thread…