Complex sentence: Nortriptyline is the active metabolite of amitriptyline which is demethylated in the liver, and which inhibits the reuptake of norepinephrine (noradrenaline), and, to a lesser extent, serotonin.
Split into simpler sentences: Nortriptyline is the active metabolite of amitriptyline that is demethylated in the liver. It inhibits the reuptake of norepinephrine (noradrenaline) and, to a lesser extent, serotonin.