It adds 4 to current and maximum health until you fire a missile, and then removes 4 from current and maximum health.
I'm proposing the change that it adds 4 to current and maximum health until you fire a missile, and then ONLY removes 4 from maximum health, leaving current health at whatever it is - then it can't kill you.
If you want it to kill you, which is fine... then i found the word "max" to be suggestive that it wouldn't, and while i don't have any better wording that isn't a lot longer (see first sentence) it's sad to die because you misinterpreted an item. Similarly, there's a buff that gives you +1 shield if you leave a level without dying, but apparently NOT if you use the levelskip consumable item, which I assume doesn't count as leaving.
That sort of thing is learnable, but annoying the first time anyone encounters it.