It’s bitter because it’s actually quite poisonous, and your tongue is warming you about that. Dave Arnold’s excellent Liquid Intelligence has a section on making your own tonic with pure quinine powder, and he admonishes the reader to mix with a blender, because an ubcombined chunk of pure quinine could actually hospitalize someone if it ended up in their drink.

Tonic water made with actual quinine rather than the synthetic stuff is great. It seems preferable to the pills, though I suspect you’d have to drink a lot.

Edit: The WHO suggest treating severe drug resistant malaria with 500mg of quinine orally every 8 hours. A 1 litre bottle of tonic water should not contain more than 83mg according to the FDA. Drinking 6 litres every 8 hours is a bit much. https://apps.who.int/medicinedocs/en/d/Jh2922e/2.5.2.html#Jh...

https://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfcfr/CFR...

https://en.m.wikipedia.org/wiki/Tonic_water

I prefer my mixed drinks with tonic over any other mixer. As you note, off the shelf tonic has very little quinine nowadays. When I had to take the pills I was headed to an area with a known malaria risk.