I don't think "this is a possible result of this treatment, as observed one time" is an illegitimate subject of inquiry because it's not a table of statistics.
If you check possible side-effects of very common medicine you can probably find some terrifying stuff that happens to 1 out of 10 million people. If it gets reported on in the media, it would seem a lot more common than it is. To be clear I'm not saying that's the case here, I have no idea.
Sure but would that make it wrong for an essayist to write about it if it happened to them? Writing about personal experiences seems to be a substantial portion of this guy's beat.
It's also worth considering that this one person's experience may represent the reactions of many. Over two-decades ago the Chicago Reader published one of the most fascinating "drug memoirs" I've ever read, and the author also had a terrible reaction to Ketamine, albeit under very bad circumstances: