Why is the effect so different?

[Pippi]

I was prescribed some Tussionex today. I have a pretty high tolerance for painkillers now, since I've been using them pretty heavily for the last couple of years. I only took 20 ml of the syrup and the effect is so diff than just taking hydrocodone. It's a very "heavy" feeling and makes me nauseous. Could someone give me any tips on why this is the case or is there something I could do to make the effect better? Thanks!