Ok, first of all MoFo's advice is horrible, don't follow it. Do not take drugs of any sort before seeing a doctor. They're professsionals, they recognize that shit with the blink of an eye.
I was just diagnosed with ADD a couple weeks ago, and yes, I actually have it. I was complete honest with the doctor about everything *except* past drug use. I admitted to trying several things once or twice and not really liking them (pot, shrooms, acid). I also admitted to trying meth once and told him that "even though my friends got high, it didn't really do anything to me so I thought it was pointless and I never tried it again." This was a lie of course; do enough of any stimulant and you will get high, regardless of ADHD. I also forgot to tell him about my regular pharmacutical abuse and the 6 month long coke and 100+mg a day ritalin habit I had a few years ago. Doctors are very reluctant to perscribe stimulants to someone they believe could get addicted, but over 50% of adults with ADD have had, or do have, a substance abuse problem. It's just part of the disease, so you're walking a tight line. Admit to some drug experimentation as a teenager, but do not admit to doing anything now or in the past couple of years. A couple more tips:
*Go directly to a psychiatrist
*Act a little figity.. Not like you're tweaking though. Stare at the things on the wall during pauses when they're writing. Sublety is the key here. Just act bored.
*See a doctor who has absolutely no relation to your current doctor or anyone that might know about your drug problems. When they ask about previous doctors, tell them you can't remember their names, you never see them more than once or twice because of your HMO.
*They're going to be looking for symptoms of anxiety and bipolor also..Know what these are and stress that you don't have them. i.e- you can't concentrate, but you aren't an excessive worrier
*be prepared for this to take awhile. they're going to make you jump through alot of hoops to get meds, this is to weed out the people who don't really need them. it took me almost 2 months, 2 psychologist, and a psychiatrist. stick with it.
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"I don't take drugs, I am drugs." - Salvador Dali