I agree with the other posters on how this would be a risky topic to talk about, but I can also see how this statement would definitely get your application noticed and set you apart from all of the other traditional personal statements in the pile.
The question then becomes, will it get you noticed in a positive or a negative way?
In answering that, ask yourself:
1. How long has it been since that incident and your previous drug usage?
Hopefully at least 7+ years, if any sooner than it might be too risky
2. What have you personally and academically accomplished since that time, have you had meaningful employment with strong LORs, great academic transcript with strong LORs? If its mediocore or below the average for the schools your targeting than I wouldn't risk it.
3. How well can you write? Would your personal statement really reflect what you've learned from and how you've grown from this? I mean the best drug addiction counselors (just say no advicits) are usually the ones that have also experimented with drugs and have seen the negative aspects of those drugs - so much that they would actually be more likely to say no then those that have never experienced them if in that situation. Can you convey this type of tone in your PS?
If after asking yourself those questions you feel that it would not be considered negative well then go for it. Although I still feel that some schools will toss your application there will be some that don't. And the schools that don't will almost inevitably call you for an interview -which is a huge advantage - as you would be a truly unique applicant in addition to them wanting to verify in person that you have matured enough and moved beyond that stage of life. If you interview well and can again convey the tone in your PS that you have matured and learned from the experience, well then you'll probably get an acceptance. Who knows you might be surprised at how many interviews a 'pull yourself up from the trenches' type of story commands.
However, to be on the safe side, if I were you I would draft two seperate copies of your personal statement, one with and one without the drug usage and then talk a paid medical school admissions counselor prior to deciding whether this is a good idea or not. I recommend writing both copies prior to advising for the mere fact that you might possibly end up in a situation were your PS could be very un-original, boring, and weak in the version in which you didn't include the drug abuse in. Which at the end of the day is just as detrimental to some schools, as you probably won't even get noticed or remembered enough to even get an interview.
