From Dark and Darker Wiki
^ when you drink a potion you get a buff 10hp heal over 10s (1hp every 1sec) = 10hp heal, for ex, how long that buff lasts is then reduced. so with -10% buff duration you'd only get 1hp every 1sec for 9sec = 9hp heal. It's different from Barbarian's Potion Chugger as it specifically states that the base duration of potion buffs are -50% before consumed and buff duration increase/decrease is applied.