I took chloroquine as a prophylactic treatment. Not against covid, obviously; against malaria. Also, I took prophylactic treatments for measles, mumps, rubella, tetanus, polio, and another dozen or so diseases. I'm currently taking vitamin D; my original justification for this was as a prophylactic against covid, but now it's just a prophylactic against hypovitaminosis D.

I had alarming symptoms after the chloroquine treatments, and regret having done it. But nobody would ever put chloroquine in the "no known side effects" bucket.