Remington Nevin is an MD and expert on the quinoline family of drugs, best known for their use against malaria. He has been interviewed here before, on the subject of mefloquine, which is believed to have caused severe neurologic damage, especially in soldiers who were forced to take it even after experiencing adverse effects. Similar effects are also found with chloroquine and hydroxychloroquine. The latter is often used for rheumatoid arthritis, and doctors there say that it is remarkably free of side effects. They forget, however, that a lot of patients stop taking it quite quickly after starting, probably because they are the sub-group that is vulnerable to side effects. Given that high doses are being used for COVID-19, and on elderly, infirm people, one can expect significant problems with side effects.
You can find out more of the work of Dr Nevin on his Quinism Foundation website: https://quinism.org