Clonidine is usually used for high blood pressure but the mechanism of action that is believed to help in pain control is different than for blood pressure control. What is thought to happen is clonidine binds in the brainstem to cells with alpha2-receptors. This binding stops the influx of calcium into the cell which in turns stops the release of the neurotransmittor norepinephrine.
Decreasing levels of norepineprine lowers the activity of the sympathetic nervous. As you know individuals with CRPS/RSD have over active sympathetic nervous syptoms and so this drug decreases the activity of the nervous system and thus pain.

I tried it but it dropped my blood pressure too much. If you use this medication as a patch and find it helpful, if you ever need an MRI you need to remove the patch for the test as the backing of the patch has metal in it.