I know it's scary the first time around because you don't know what to expect. I don't know about opiates. I've had 2 of my babies with no drugs, 1 with pethadine and the other 3 with only gas.
I would suggest that at your next visit with the midwives, you speak to them about your pain relief options. They will be able to give the best advice for your situation, and you'll be able to go home and consider your options before the big day.
As every birth is different, I always have gone in with an open mind. If I can manage through the contractions with breathing and focusing techniques, then that's great. If that's not working then I'll ask what are my options for the stage of labour I'm at. Some drugs like pethadine, you can't be given too late in labour as it does pass through to the baby. Perhaps you could ask about an epidural. I have never wanted to use this form of pain relief but it might be a good option for you.
Most of all, don't worry yourself too much about it. You'll do fine and like most of the rest of us, you'll probably go back and do it all again one day.