Pharmacologist Ian Musgrave notes a rare death in the UK:
In this particular case, the deceased had consumed an entire tin of caffeine-containing mints. Each individual mint contained 80 milligrams of caffeine, about the same amount as in some moderate-level energy drinks. Consuming the whole tin of the mints is like consuming 12 cans of a moderate-level energy drink, one after the other. But is that enough to kill you?
The handy website Death by Caffeine, where you can find out how many cans of energy drink, cups of coffee or bars of chocolate you will need to consume before expiring, suggests that a 70-kilogram [154-pound] person would need to drink 132 cans of a beverage containing 80 milligrams of caffeine (or a similar number shots of espresso coffee) to die of an overdose. If that’s correct, then [the victim] should have had a tenfold safety margin. So what went wrong?
The reason is related to why dogs can’t eat chocolate:
Caffeine (and the related stimulants from tea and coca, theophyline and theobromine) is broken down in the liver by a specific enzyme (cytochrome P450 1A2 for the technical-minded). Not everyone has the same amount of this enzyme in their livers for many reasons, such as the gene for the enzyme being missing or defective. The reason you don’t give chocolate to dogs is that they have very low levels of their version of the human enzyme and are more susceptible to toxicity from theobromine and caffeine in chocolate.
Around 40 percent of Caucasians have a version of the enzyme that breaks down caffeine slowly. In these people, caffeine consumption is correlated with higher incidences of heart attack and high blood pressure. But in this case, the reason was not a genetic variation but disease. The deceased had cirrhosis of the liver, which, among other things, greatly reduces the ability of the liver to break down a variety of chemicals, including caffeine.