In general, the level at which caffeine becomes toxic is when you've ingested 10 grams (the equivalent of 50 to 100 cups of brewed coffee, depending on the strength). However, as PopSci explains , the problem is that the exact caffeine limit for you depends on your weight and the way you're getting that caffeine (e.g. in a Starbucks Frapp versus an energy drink). The weight factor makes sense — it's similar to the way that the same amount of booze can make differently-sized people feel very different levels of drunk. And if you're puzzled about why the delivery method matters, it's due to the level of dilution of caffeine (more water means it's more diluted, which means you'll have to drink more of it to get fully caffeinated) and what else might be lurking in that drink, which may be other stimulants or even alcohol (remember Four Loko ?).