Norepinephrine. AKA Noradrenaline.
Both ritalin (and other ADHD stimulant medications) and caffeine stimulate the production of norepinephrine in the brain, and norepinephrine, which is also implicated in depression, is identified as the we-still-don't-know-how-it-works factor in how ritalin, um, works.
From which you can draw the hypothetical conclusion (and you wouldn't be the first) that many caffeine addicts are self-medicating for ADHD. I have even seen reports from mothers who noticed behavioural improvements in their children after including caffeine-containing substances in the normal diet. In fact, the children even requested their "food medicine".
Now, I work with the public and there is no way in hell I am going to take a self-report like that at face value. But I also work with brain injury and there is no way in hell I am going to deny the possibility of ADHD pathology being the result of biochemical processes in the brain, so if anybody feels like arguing that ADHD is just a result of parents not being able to cope with kids who misbehave: Don't bother unless you have evidence.
Which is the problem. We don't yet have a diagnostic test for ADHD which is not behavioural, so diagnoses are never going to be, in these situations, absolutely trustworthy. Then again, we don't have a diagnostic test for schizophrenia or depression which is not behavioural, either - we base our diagnoses and our definitions upon patterns of behaviour which occur so consistently in each other's presence that we may as well give in and label it. A good clue to whether or not a diagnosis is correct is if the standard medication results in the desired improvement. If it doesn't, then you may have to face the fact that you're a crap parent, or your kid has a food allergy, or they're acting up at school because they're smarter than their teachers and they're bored rigid.
I'm placing my bets of EEG or fMRI studies being the most promising.
But, being a heavy caffeine user who is prone to depressive episodes, has difficulty staying on track, terrible time-management skills and a tendency to have periods of low-energy-crash early morning or afternoon, the thought that I might unintentionally be self-medicating I found rather amusing. So I went looking for self-diagnostic tests, because I was supposed to be doing something else and therefore clearly wasn't concentrating properly.
From the sarcastic to the suspiciously short, there are a plethora out there. I found one which seemed worthwhile, not only for comprehensiveness but for what it says about the behaviours which are regarded as "problematic" (clue: you've probably got most of them - that's what you get when you list almost everything). And yes, it does say IMPORTANT: This is not a tool for self-diagnosis at the top. Naughty me.
My results? I can't see a measure for what the total score means, but based upon the general pattern of responses I show definite clusters of ADHD symptomatology in several areas, and an overall strong likelihood of being affected.
So I wrote myself a prescription to visit the percolator. Bottoms up.