If you have chronic pain, or any kind of intermittent health problem, then you should join Twitter.
Why? Well, in case your last trip to the hospital wasn't a clear enough sign, the communication breakdown between doctors and patients is costing us dearly. Too little treatment, too much treatment, improper treatment, it all boils down to the fact that your doctor doesn't know enough about you to make some of these calls.
What were you doing when the pain struck? Getting out of the car? Well, what did you do before that? An hour before? Four hours before? Last week? Did you move furniture? Sit at your desk all day? Or did you go for a run? If you used Twitter, all this information would be available for your friends, family, and even doctors.
Naturally, this does open up the problem of information overload... and in fact studies have shown that doctors can sometimes give better treatment by asking fewer questions. The most famous example is probably the chest pains decision tree described in the book Blink: by asking three questions and no more, doctors have a better chance of catching heart attacks. Of course, that's just for determining whether your chest pain is serious enough to warrant hospitalization, or whether you should take a Tylenol and go home. After a patient is in the hospital, the more information your doctor has, the better your treatment will be.
By constantly recording what you are doing with twitter messages (aka "tweets"), you give your doctor a wealth of information about your habits. If you moved furniture three days before you felt pain, you might not remember... but its in your Twitter feed, so your doctor will know.
Why Twitter and not a diary or blog? Simple... the easier it is to do, the more likely you are to do it. Diaries and blogs are for long-winded thoughts, rants, and essays. Twitter is simply what you are doing right now, and nothing else. Its clean and simple, and you can even "tweet" via cell phone text messaging. Nobody expects you to be pithy, funny, or even interesting... just approximately accurate.
In short, if you want good medical treatment, you should tweet your pain.