Is the Growing Boy perk only applied once and when to use?

In Stick of Truth there is the perk Growing Boy (Your maximum health increases by 20%). Is this perk only applied once or is it applied with each level up, when the maximum health increases?

Either way, should it be used in the early game or in the late game?


I haven't played the game in a long time, but from memory, this particular perk adds 20% of your total health at the time you buy it.

Either way, even if it's more useful to buy it in late game, I'd recommend buying it early, because you'll need it, especially since combat diffuculty scales up quickly.