Erowid puts the LD50 for Codeine at arounf 700~900mg. Considering some of us take 450mg recreationally (300mg myself), its not a huge jump to the LD50, and even if you didn't die, you'd still get pretty unpleasant effects once you got close to LD50 (since 50% die before LD50, its highly likely you'd get at least some seizures).