What's a Flask?
Previous ARPGs, particularly Diablo II, used a consumable potion system that ended up having two main problems. One, you ended up spending a lot of time just picking up potions from the ground. And two, you could often trivialize a boss simply by filling an inventory with potions and chugging them one after another mid-fight to the point of near invulnerability.
Path of Exile solves these problems by not having potions at all. Instead, it has Flasks, which are effectively reusable potions that can carry modifiers like other items.
How They Work
Flasks are equipped in five slots located near the bottom-left of your screen. While in those slots, they will slowly fill up with Charges as you kill enemies (or will refill entirely anytime you visit a town). Using a flask consumes some number of its Charges in order to provide you a temporary benefit.
Most Flasks can hold enough charges to be used more than once, but each use depletes them, so you cannot simply chug flasks in a boss fight.
Some effects can add extra Flask charges. The most notable of these, which causes you to gain charges when you critically strike, has a rather substantial 0.2 sec cooldown before it can grant another charge, so more than one such effect is usually not useful. Effects that grant charges in other ways usually do not have such a cooldown, though.
Flasks themselves are not consumed on use. As a result, Flasks can have Affixes just like equipment can: a Flask can be Normal-rarity, Magic (blue), or Unique (orange), although not Rare (yellow). Affixes on a Flask can affect how many charges they gain or consume or apply additional effects while the Flask is active. In general, modifiers on a flask are local and affect the flask itself only unless otherwise stated.
Types of Flask
Flasks come in two main categories:
- Life Flasks and Mana Flasks restore a fixed amount of Life or Mana over a duration. If your Life or Mana fills during the duration, the effect ends immediately, and won't restart even if you again dip below maximum Life or Mana within the duration for which they'd normally be active. There are also Hybrid Flasks, which restore lesser amounts of both Life and Mana.
- At the higher tiers of Life and Mana flasks, there's not a strict "this flask is better than this other one" hierarchy. Some high-end flasks heal more, some heal faster, and some heal more per Flask charge, but they're not strictly superior to one another - it's a trade-off between these different things you might value in a flask.
- Utility Flasks, which provide a variety of utility effects for a fixed duration, including (but not limited to):
- Quicksilver Flasks, which increase movement speed.
- Quartz Flasks, which allow you to move through enemies.
- Ruby, Topaz, Sapphire, and Amethyst Flasks, which increase your Resistances to Fire, Lightning, Cold, and Chaos respectively.
- Bismuth Flasks, which give less resistance than Ruby/Topaz/Sapphire flasks but affect Fire, Lightning, and Cold resistances at once.
Life and Mana Flasks come in many different tiers, which you'll want to upgrade periodically as you level (and if you have not already done so at this point in the guide, start looking!). Utility Flasks do not, and you may very well be using a Quicksilver Flask obtained five minutes into the game all the way through the main storyline.
Using Flasks Properly
The first, and most important, rule? Don't chug Flasks! Even if you want to keep their effects active constantly, which you often do, you want to reuse them only when they're close to expiring.
You will almost always want at least one Quicksilver Flask in your belt, often two. Quicksilver Flasks keep you moving quickly, and reducing downtime is a big part of maximizing your drops in PoE.
Against trash packs, you can usually just keep your flasks active throughout the fight - you'll get so many flask charges back from kills in any reasonable build that you're likely to be able to maintain enough charges to use them continuously.
Against bosses, be sparing with your flasks, unless you have some way of recovering charges. While during the story you can take a Portal back to town (and thereby restore all your Flask Charges), you won't be able to do this in endgame Maps at no cost (because maps limit the number of times you can enter them, and leaving/returning consumes one such entry).