Magnesium sulfate / potassium sulfate / sodium sulfate
"So far so good.  I followed the tips to drink it very cold, with a wedge of lemon handy (Best. Tip. Ever.) and through a large straw.  It was nasty but drinkable.  I had an immediate reaction and apart from feeling a little gross for about an hour after drinking it, it was no big deal.  No, I'm not especially looking forward to the next bottle but it's not the worst thing I've had to do.   Comparable to warm barium if you ask me."