When a creature successfully drains you of blood or some other life force, you become less healthy. Drained always includes a value. You take a conditional penalty equal to the value on Fortitude saves and Constitution based checks. You also lose a number of Hit Points equal to your level (minimum 1) times the drained value, and your maximum Hit Points are reduced by the same amount. For example, if you’re hit with an effect that inflicts drained 3 and you’re a 3rd-level character, you lose 9 Hit Points and reduce your maximum Hit Points by 9. Losing these Hit Points doesn’t count as taking damage.
In most cases, the drained condition heals naturally at a slow rate. Each day, when you regain Hit Points by resting, your drained value is reduced by 1. This increases your
maximum Hit Points, but you don’t immediately recover the lost Hit Points. When the drained value reaches 0, you no longer have this condition.