Most magnesium salts are poorly absorbed from the gut into the bloodstream and brain via transporters, and high doses can cause diarrhea.

“most magnesium salts don't passively get from the gut into the bloodstream, into the brain. They pass via what's called a transporter. ... if you take a normal magnesium supplement that you can buy at the pharmacy, it doesn't cross the gut very easily. And if you would take enough of it to get it in your bloodstream, you start getting diarrhea.”