stacking flask usage ?

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LordOfShadow wrote:

Let's say I have a life flask that recover 4000 hp over 10 sec.
let's says this flask have a cooldown of 2 sec.

Flasks do not have cooldowns, but that's irrelevant to the question anyway.

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LordOfShadow wrote:

If I use that flask, I will recorver 400hp/sec, ok.
And if I reuse the same flask 5 seconds after the first use, do I have 800hp/sec (during 5 sec) and 400hp/sec for the last 5 sec ?

No. Flasks don't stack, they queue. If you use that flask twice, you'll recover life at a rate of 400 per second for a total of 20 seconds (or stop before then when you reach full life).

If you used two different flasks, and the second one had a faster rate of life recovery per second, that one would go to the front of the queue and the in-progress slower first flask would wait until it finished, then restart and do the rest of it's recovery - flasks are prioritised by recovery rate.