Gracie,
This has been a really great thread. I wanted to add one more thing. Whilst I never took pethedine and never even thought about it, I can tell you that based only on what others have said, OFTEN IT DOESN'T DO ANYTHING FOR THE PAIN!! Many women say that all it did was make them, and the baby, very sleepy and unable to focus.

Natural endorphins will kick in during labour. During my 2nd VBAC, because my ds had his elbow up by his head, I felt a lot of pain during contractions. Between contractions, I actually slept--even though it was daytime and I had no lack of sleep. I felt silly for constantly nodding off, but the midwife assured me that it was endorphins at work. Had I had pethidine on top of that, I think I would have been comatose.
Also, the good thing about your natural endorphins is that they are produced just as you need them. They don't have to be topped up and they taper off when no longer required. Once my son pulled his elbow down, everything was much more manageable and I was immediately far less sleepy. I was then wide awake and feeling fabulous during stage II, which was very exciting.