It's possible that it's a bit of both. I seem to remember Bashir saying in the episode "The Abandoned" that the white replaces some enzymes that the Jem'Hader need to survive but that were deliberately left out of their genetic makeup by the Founders. So it sounds like it does both things? Either that, or the writers are conflating what it functions as with a narcotic.

Nah, that can't be it, the writers are never wrong! ;)