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Thank you all for your answers and - some horrendous - experiences, especially Matheous-2, I'm sorry your wife had such a bad time on Gabapentin. I've been taking it for a few days now and it seems to be doing nothing for the pain. I've had kidney, hip and leg pain for 11 years, plus I get Costochondritis every year, always in February, which lasts for several weeks, so all of the pain is in the same place and sometimes it's too much to bear and I want to scream!
However, a couple of the side effects have become rather convenient. I feel almost permanently 'stoned' and I seem to have boundless energy. As long as I take things slowly, I'm getting more done in a day than I have in years. It's very odd! I'm falling asleep for a couple of hours around 11pm/midnight, so I'm waking up at 2am-ish, which is a bit annoying, but I've just done all the dusting. My ribs and kidney are aching, so I've sat down and now I'm on here for a while. If the pain gets any worse, I'm going to lie down and read for a bit.
Many thanks once again for your answers, it was interesting to read about other people's experiences and it's amazing how they differ so radically!