How come I am still alive?

I took approximately 420 mg of methylphenidate(ritalin, concerta, mythlin, etc) all at once and didn't die. It was taken orally also. I had been taking the perscribed amount since the first grade. I took this high dose once when I was 15 one time and I'm 17 now still alive. I just remember feeling really high and tweaked out for the whole day. I have read that doses in the 200 mg are extremely high doses. I wasn't trying to kill myself either, I just wanted to get really high. I had no idea that 420mg was such a high dose. So why didn't I die?