He got Ketamine in season 2, which worked for his pain...for a while, anyways.
How House didn't keel over dead from taking that much Vicodin/wasn't chronically constipated(a side effect) I will never know. There's a great scene from the episode House Divided. They're tasting alcoholic ice creams for Chase's bachelor party. House scoops some ice cream and puts a Vicodin in it, then eats it.
My friend from Germany and I were marvelling at the fact it's been almost a year since the finale.