https://astralcodexten.substack.com/p/ivermectin-much-more-than-you-wanted
A couple of months ago I thought about wading into the research on Ivermectin to understand if there was anything approaching a scientific consensus around its use in the treatment of COVID, and I quickly got discouraged by the large volume of contradictory studies of varying quality as well as the low quality of discourse on all sides around it (eg "Ivermectin is the wonder drug Big Pharma doesn't want you to know about" vs "LOL, look at these idiots popping horse dewormer", neither of which is very useful).
Fortunately, Scott Alexander was willing to put in the work to really scrub this research hard and become an expert in what's there. He went through every one of the 30+ studies cited in discussion around it. He was able to remove the ones that were poor study quality or outright fraud and then look at the rest. The overall picture shows a correlation between [the location of studies showing Ivermectin is effective] and [the rates of worm-related infections in that area].
This suggests a theory that ivermectin is effective against COVID when the patient is also dealing with a worm-related infection. This makes sense, as worm-related infections impact the function of the immune system, some COVID drugs suppress immune responses that fight worms, and the last thing your body wants to be doing when fighting COVID is having to also deal with a rapidly multiplying worm population. Thus, it could be a great idea to take ivermectin prophylactically or upon COVID symptoms if you are likely to also be dealing with a worm infection. (FWIW Scott assigns only a 50% probability that the worm theory is true; it's so intellectually compelling that it's hard to let go of it, but so far the evidence is correlational.)
This brings up an important general point that medicine needs to be local and targeted at the specific situation, and to do so we need to understand the actual causal mechanisms for things. What may be true for a Bangladeshi population in Bangladesh may not be true for a Bangladeshi person living in the UK, a Black person living in the UK etc.