Here's how mindfulness changes karma. When you sit (in meditation) you are not allowing your impulses to translate into action. For the time being, at least, you are just watching them. Looking at them you quickly see that all impulses in the mind arise and pass away, that they have a life of their own, that they are not you but just thinking and that you do not have to be ruled by them. Not feeding or reacting to impulses you come to understand their nature as thoughts directly
So, who here meditates? It's a great way to look at the thoughts which arise related to addiction. It also gives you a new perspective on life and yourself. I highly recommend this book if you want to start. There's an app called Headspace which has many guided meditations and is really good. It even gives you tit-bits of wisdom during the day by way of mindful updates.