Answer by Robert Rister:

It happens. Some people don’t have a lot of the liver enzyme CYP1A2  that “activates” alkaloid stimulants, such as those in coffee, or methamphetamines. The 60 grams of sugar is not as much as it sounds like. It’s not so much that your body is suppressing the function of the drugs as it is just not responding to them, not because your nervous system is different, but because your liver is. Incidentally, cruciferous vegetables (Brussels sprouts, cabbage, that sort of thing) and charcoal-grilled meats would activate the CYP1A2 so caffeine would give you more of a kick, as would several dozen other common foods and medications. Citrus fruit blocks it, and blocks the effects of caffeine.

Why don’t stimulants, specifically caffeine and amphetamine, affect me?