Although not intended for ADD, Dextromethorphan (the common drug found in cough syrups) is good for treating ADD in moderation. A good dose would 100-200 mgs, depending on the severity of your ADD. I wouldn't go above 300, unless you planned on getting completely stoned and stripped from reality. This medicine is a dissociative and helps you focus a lot better on one particular task (from my experience). I'd take it only once daily; probably when you first get up. Your most intense effects will kick in about an hour upon consumption, and will last 4-5 hours. The aftereffects, however, should last the rest of the day. Take this on an empty stomach, but be sure to ingest some food after taking it (but not a whole lot).
What's best about this? It's over the counter, not prescription. Just make sure the only active ingredient when you buy the syrup (also comes in gelcaps) is Dextromethorphan hBr. I'd actually just start on 50 mgs just to be safe. If that doesn't work for you, increase it by 50 the next day. If it still does you no good, increase it again by 50 and etc. This isn't like an antidepressant where it builds up in your system over time. Like alcohol, weed, prescription opioids, etc. it hits you very soon after consumption.