Personally, I've found that I can go for days without doing anything useful if I am trying to avoid boring work. I feel really tired and generally run down. The way I combat this is to do something interesting, and let the momentum help me do boring things. (Video games also seem to help, a cup of coffee and an hour of Geometry Wars puts me in a great mood.)
Anyway, if I were going to go on drugs, I would do it in a structured way. Don't medicate yourself, get a doctor. Before you start taking anything, get in the habit of keeping a diary. Write down what you are thinking, how you feel, ideas you have, what you did with the day. Then, start your medication. Keep writing your in journal. If something changes, it should be easy to see. Your drugs might not make you more useful, they might just be lowering your standards. (My friends think they are much more interesting than they actually are when on certain recreational drugs.)
Finally, maybe your standards are just out of whack, and you are imagining things that aren't true. I often feel inadequate, even when I consider that I have written 100+ pieces of software that people actually use (with about 10 maintained at any given time), I've written a book, I write a moderately popular blog, I am invited to give talks around the world, and have high HN and StackOverflow karma. I think we always feel that we can do better, but that's not a medical condition, that's just ambition. You should stop to think about whether you are actually doing as badly as you think you are.