> (For the record, 3 half-litre beer steins of coffee a day turned out to be a spectacularly bad idea, even if hearing colours was briefly entertaining.)

I've consumed caffeine to the point of serious gastric distress and very mild heart palpitations and I've never experienced synesthesia. I'm unwilling to push beyond that to higher doses, but two or three 12-cup pots a day of light roast filtered black coffee is about average for me, so what the Hell? Is there just that much individual variation here?

IIRC, the only well documented case of synesthesia is https://en.wikipedia.org/wiki/Grapheme_%E2%86%92_color_synes... . It's an interesting case because the color and letter recognizing parts of the brain are very close, so it's logic that some info may leak and get correlated.

Perhaps someone can design an (safe) experiment to measure the effect of caffeine in synesthesia ...

There must be. At a previous job, I regularly drank 2L of coffee a day (in ~10oz increments), and never experienced anything other than unpleasant slight jitteriness occasionally.

I think my cups are closer to 200 ml, which apparently is approaching 100 mg caffeine per cup, so I guess you're right.

A "medium-size coffeehouse coffee" (at Starbucks) is 16 fl.oz. Here in Europe, that's not really a normal cup of coffee.

America! What's a normal-sized coffee there, to compare? Large at Starbucks ("venti") is 20oz (~591ml). The caffeine levels at $ seem to be similar between USA and EU: http://www.caffeineinformer.com/the-complete-guide-to-starbu... but I'm guessing that that's more due to it being such a touristy place. From what I've seen, they use less espresso than "real" coffee shops: http://www.starbucks.co.uk/blog/introducing-our-new-standard... if you've paid attention to espresso machines for whatever reason, you know that most machines make two shots per pull, so typically higher-end places will use either 2 or 4 shots (since that's either 1 or 2 times they need to make espresso for the drink). If they have a large size (16-20oz), I've found that it's common to use 4 shots in that, which is going to be more caffeine than $, possibly by a wide margin.

If you're in the Bay Area and really want to go nuts with your caffeine for whatever reason, Dutch Bros in North Bay / East Bay (usually go there when I'm by Sac) has drinks on their menu with 6 shots: http://dutchbros.com/AboutUs/OurMenu and they're atypically strong per-shot.