Melatonin.

[He takes the bottle, carefully turning it over in his hand. Luckily, that word is currently burned into his brain.]

I went back to the Nurse's Office, and it looked like someone had been there. There wasn't any blood or anything like that, but the pill bottles and pain killers had been rifled through. It didn't look like anything was missing...

But I checked the supply list. A bottle of melatonin was missing from it.