I had 2 shots of pethadine when in labour with my daughter and I found it to be good at first. It didn't seem to take away all the pain but enough for me to rest in a bath for about 8 hours!
I did find after a little while that it made me feel giddy and I felt dizzy too. After the second dose I was so woozy that I could not move around because if I did I felt like I would throw up. It did wear off eventually and I felt less in a haze if you know what I mean. I don't think everyone experiences the same reaction to it that I did, however I have heard that a common side effect is nausea.
Hope that doesn't put you off at all - that is just my experience.
Good luck with the birth.
4 lovely munchkins DD 10/03, DD 03/05, DS 10/06 &