Caffeine, which I normally consume via coffee, has the usual physiological effects on me. Unfortunately, the magnitude of these effects seems to vary randomly. Some days, I can have two cups of coffee and be fine - a little more alert, perhaps, but otherwise no significant variation relative to uncaffeinated. Other days, like today, a single cup of coffee renders me jittery and (more problematically) drives me to the washroom at frequent intervals.
The frequent-washroom thing is actually a bit more predictable than the alertness/shakiness effects - so you'd think I'd have learned by now not to drink coffee before or during a long meeting or lecture.
Today, Patrik Nosil defends his PhD thesis. I bought a coffee just before going, and I was perfectly OK through his 40-minute presentation (which was excellent, by the way). During the break between the presentation and the start of committee questions, there was a short break which I took advantage of - thinking, erroneously, that this would allow me to sit through the 2 hours or more of questioning. Wrong. I had to bail because of my traitorous, unreliable excretory system. Damnation!
Oh well, perhaps someday I will become familiar with my own damn physiology.