Unfortunately, our society has put some serious stigma on the issue of drug addiction. I think that others are right that you should not bring it up, however, it is pretty admirable that you were able to overcome that and do well enough in school to apply to med school.
On another note, make sure you have some serious "clean time" before you enter med school (as in, at least a couple years). For one, med school is stressful. Secondly, you will be around drugs, although legal, and at some point you will have the power to prescribe them.
Good Luck