Yes, Elden Ring allows you to use your Flasks during PVP combat. Whether you’re invading, being invaded, or challenging someone to a duel, your Flasks will be available to use during the fight. However, some players will avoid using their Flasks in PVP out of courtesy to their opponent. In general, it’s polite to refrain from health-recovery items, but since using FP is required for so many different builds, most players won’t have an issue with their opponent sipping a Flask of Cerulean Tears so the battle can continue.