Water. It's absolutely essential for every single cell in your body. You can swim in it, wash your windows with it, shower in it, do the dishes in it and paint with it. But are you drinking it?
Many of us are constantly dehydrated without even knowing it, because we're drinking only coffee, soft drinks and other beverages that actually dry out our cells. Partial dehydration leaves you drained and tired, and has a long range of effects that may often aggravate or intensify chronic pain. Also: if you are taking medicines for your health condition on a daily basis, you need plenty of water to flush toxins from your inner organs.
As a chronic pain patient you should drink more water than the average recommended 8 x 8 oz glasses a day (2 liters). My pain specialist adviced me to drink 3 liters a day, and that seems to work really well.