So hydroxychloroquine (brand name Plaquenil in the US) can have some pretty gnarly side effects. It has a long elimination half life, meaning that it sticks around in your system for a while after you stop taking it. This normally isn't a big deal, except for some reason, the cells in your retina can not void the compound like the cells in the rest of your body. The reason for this is still poorly understood. The levels can get high enough that your retinal cells start to die. The peripheral vision starts to go first, resulting in ever-narrowing tunnel vision.
Hydroxychloroquine-induced retinal toxicity is permenant, and can contiue to get worse even after cessation of the drug. For this reason, there is usually a lifetime exposure limit of hydroxychloroquine. Patients are switched to a different drug after they get to this point, and they have to undergo regular visual field testing and retinal scanning to keep an eye on their eyes.
This sounds bad, and it is, but hydroxychloroquine is still a lot safer and more effective than some other drugs for these conditions. This is a rare side effect that only a few groups of patients have to worry about. The people who are most at risk of this are on a higher dosage for a long time, or they are dumb assholes buying it over the internet and taking an inappropriately high dose. Medical supervision is important while taking this drug.
For all you assholes in this thread who are buying this shit from the internet without medical supervision, don't take more than 400mg a day as a maintenance dose. If you are under 135lbs, your safe dose is lower. Tell your idiot family members who are getting their fake scripts from pill mills online to not overdo it on the dosing. More is not better. I'm waiting for the rash of tunnel vision and legal blindness in a few months/years to come as a result of people ODing on this thinking it will protect them from corona.