Only 40%? That surprises me.
Most of the ones I've consumed don't cause a problem as long as they're done sparingly. An 8-ounce Red Bull pretty much does the job it says it does on the label. Rockstar seems to be the most effective at keeping people awake smoothly.
But Monster? Blech. I've had strange reactions every time I've tried one. Strange heartbeats, shortness of breath, angry/agitated mood... I ended up tracing it to L-carnitine, an ingredient Monster uses that other drinks don't.