I would assume you either keep the title, or the title becomes vacant and you face a mandatory for the belt. I can't think of a specific example to back this up, just thinking out loud.
I don't think much would change the result would stay the same however it would go on record that that fighter used an illegal substance. However if the fighter that won failed the drug test i could see the fight being made a no contest and the belt being given back to that fighter or vacated depending on the situation regarding that championship.
Doesn't get made a no contest the result still stands if the loser fails a drug test. I.E. Bradley vs Casamayor.
If you are Campillo you get stripped from your belt. For sure. If you are Sexy, the Wbc strips your belt for the second time.
That's not the rules. They don't punish the winner of the fight because his opponent cheated. Just shows how long you HAVEN'T watched boxing.