Every medication has a side effects and you balance if the benefits are better than the side effects it causes. Chloroquine has tons of side effects[1] though (including things like causing trouble with breathing). It might still be worth it if you have Lupus.

When an existing drug is intended to treat some new disease it needs to be reevaluated for that disease again, because even if it can save lives for one disease it can be deadly for another.

So the fact that it is given to thousands or millions does not mean much in this context.

[1] https://www.drugs.com/sfx/chloroquine-side-effects.html