This makes me wonder if it would have been possible to, instead of using something that causes significant harms to health, instead something which just causes a rather unpleasant experience.

But, I guess one important thing is whether such an additive is compatible with the altered substance still being usable for the purpose for which it is being made available.

Now, presumably something as simple as “dissolve large amounts of capsaicin in it” wouldn’t work (I’m not even sure if capsaicin can be dissolved in alcohol), but, what about something along those lines? Or something that just causes headaches, or vomiting?

During prohibition, the government tried things along those lines, as well as basically any poisonous or unpleasant chemical they could get their hands on. However, the problem was that anything not sufficiently alcohol-like could be pretty easily distilled out by bootleggers. What was much harder to distill out was large quantities of exceptionally-poisonous methanol.

Nowadays, of course, alcohol is cheap and plentiful enough that there's no incentive to re-nature it, and we can get away with much less extreme measures. In many cases, the alcohol won't contain any methanol at all, and may even be drinkable.

A good article on the government's arms race against bootlegging: https://slate.com/technology/2010/02/the-little-told-story-o... (also, the author's book "The Poisoner's Handbook")

They already add a bittering agent to ethanol when it's sold as a disinfectant. It's not toxic, just very unpleasant

Not true, they add a bitterant but if it's in the US they're still required to add acetone and methyl isobutyl ketone as well.

https://www.law.cornell.edu/cfr/text/27/21.49

That formula clearly spells out rubbing alcohol and disinfectants under the authorized uses. It's toxic and unpleasant.

Woah! I think it's different here in canada then.

I think that's what they ended up doing, post-Prohibition. A bottle of rubbing alcohol from the drugstore is either isopropyl alcohol, which is toxic by itself but not intentionally so, or ethanol mixed with a denaturing agent that's more benign than methanol or whatever they were using back in the day.

Different denaturants for different purposes.

Choose your poison:

27 CFR § 21.151 - List of denaturants authorized for denatured spirits. https://www.law.cornell.edu/cfr/text/27/21.151