It supposedly causes the taker to see flashes of light that aren't there.
And possible brain damage. The stuff may have prophylactic properties but I don't trust it. My sister and her husband have been knackers since they started taking it. They have been taking it for a couple of years now. By knackers I mean very weird. They both have said some very bizarre things. They used to be scholarly and intelligent. The husband has had some very strange autoimmune things pop up too.
I just keep myself well nourished and eat foods known to boost the immune system. I do think the government blew the Covid stuff way out of proportion and they didn't manage it properly.