N-desmethylmethsuximide

What it isActive N-demethylated metabolite of methsuximide; a succinimide T-type calcium channel blocker.

Why it mattersA half-life of about 33 h against the parent’s 1.4 h makes it the circulating form, and the level to follow.

What it changesIt explains the biphasic overdose: patients wake, then relapse into coma within 24 h. Levels above 40 µg/mL have caused toxicity; coma at 150 µg/mL.