I found out with my second birth when I ended up in the hospital due to not being able to line up a home birth in Romania, it got so hard that I was like, I'll take meds now. I knew I didn't want them before, but I couldn't control myself. Thankfully, they didn't have any. Said I had to get them at 3-4 cm or I couldn't have anything. Weird, but it worked out. haha
But, now that I've done it all natural twice, I suspect I could be stronger if I had access to pain meds, but when they dangle the "candy" in front of your face at the hospital, oy.