https://en.wikipedia.org/wiki/Magnesium_citrate

In solution with water it tastes somewhere between the worlds worst flattest sprite and a sweet lemon.

Edit: I'd marked Sodium Citrate, which is a similar compound but is used for different reasons. Sodium Citrate is an anti-coagulant. If you've ever donated blood plasma, its that weird sterile taste you get in the back of your neck when they feed the blood solids back in with saline. It is also used in nacho cheese.

Before a colonoscopy, you typically have to drink something called "prep" to clean out your system. For the better part of a day before the procedure, you drink nothing but this liquid that is meant to clean out your intestines, and it makes you shit your guts out until it's nothing but the clear liquid. It's usually lemon or pineapple flavored, and it tastes awful.