This study has some interesting conclusions about pain relief and its effectiveness in labor. But I would argue that it can be true for any type of pain relief. Anyone who has migraines can tell you that there is a window of time during which you need to take the medication or it will not work.
Of course I don't want people to have to use pain meds during labor anyway... And I blame technique and medicalised birth for the use of pain meds as the default. I mean, you could help a mom by getting her on her feet, by changing her position. Maybe if you didn't insist on women being flat on their back and strapped to a monitor they wouldn't need the meds. But that said, the less medication they have to give women who request it the better. Any BFing support person will tell you that medicated babies are more prone to problems with breastfeeding, no matter what the Drs say. And, any woman who has ever had a spinal headache or one of the other unwanted effects of an epidural probably doesn't remember the "relief" it gave her at all.
OK, stepping off the soap box. Just wanted to share this article:)