If you have arrests for drug possession/distribution, you're going to have to discuss it at some point in your application, whether it's in your PS or an addendum. If your arrests are not for something drug-related, and there's no way they would know you're a recovering addict, I wouldn't bring it up. I agree with Johnny- they're conservative and it would be really risky to do.
But, if your arrests are drug related, you have to talk about it anyway. I don't think it matters all that much if you do it in your PS or an addendum, but if you have a better story for your PS, I would go with that one for sure, and save the drugs for an addendum.
Also, I'm gonna give you the same advice as I'm giving my younger sister (who also has a possession arrest/no charge)- take some time off and do something to show them that you've moved past this part of your life. Work at a rehabilitation center, do peer counseling, something to concretely show that you have learned from your past and have truly changed. It's great that you're making good grades now- but I think really you'll be fighting an uphill battle and you need more than good grades to make up for your arrests.
I'd be happy to answer any other questions you have about this via PM, but I think what I said already is pretty much my best advice.