You can use it anywhere, but like you said your HP drains, that's the effect the skill has. (If it didn't, it would be way too overpowered.)
After your HP became low, simply relog and eat GJ, then you can use it again.
Or wait untill your HP reached 100, and you'll return back to normal automaticly (beware because it has a bug.)
Sidenote: You can't swap equips, use items, use skills, or getting healed in Frenzy.