Comment by DaveZale

Magnesium (Mg²+), the natural NMDA channel blocker, has an apparent dissociation constant (Ka/Ki) of roughly 2-3 mM (2,000,000-3,000,000 nM), making memantine ~1000x stronger at NMDA than Mg2+.

Magnesium sure wears a lot of hats in our biochemistry. It's also weakly active on our benzodiazepine receptors. And involved in dozens more areas, or more