User blog comment:UnboundIsaac/Oh No!/@comment-24058253-20150419195115/@comment-24058253-20150420205032

I know this is true for potions, so if a lesser healing potion is left of a healing potion, the lesser is used first, or if a restoration potion is left of a healing potion, the restoration gets used first. This is of quite some importance to storing potions in the inventory.