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As someone who just had his done, I make a few recommendations:
1) Go for the full colonoscopy. I say this because if you are having blood and the sigmoid shows nothing, you'll have to have the full deal anyway. PLUS, with the full colonoscopy you'll get drugged up and it will be more comfortable.
2) Make sure your doctor KNOCKS YOU ALL THE WAY OUT. Mine didn't. It hurt a couple of times. (At one point I shouted a stream of ex-pat New Yorker curses in to the face of the very sweet small-town nurse who I thought was going to faint from shock.) But that being said, on the pain scale it actually hurt less than stubbing you pinky toe on a cold morning...just creepier.
3) If you're someone who *ever* worries about finding blood, a colonoscopy will totally help you sleep better at night.
If you're over 40, just get it done. A clean result means you won't have to worry for another decade, and Katy Couric will no longer make you feel like a wimp.
Andrew