Fentanyl as a painkiller during giving birth

I have discussed painkiller options with a hospital doctor where I'm planning to give birth. She said that sometimes it's too late or too early for an epidural so they can give an injection of fentanyl if I wish. The effect lasts 45 minutes. Has anyone tried it? Is it dangerous? What's it like? Does it remove the pain well but leave you conscious enough to follow doctor's instructions as to how and when to push etc.? Will there be a "withdrawal" situation the next day?