In my experience, castor oil can definitely help. I don't remember how much I took, but it was a "half dose" based on what I'd been reading before I did it. My first son had been 11 days past his due date, and due to his complications and resulting death, my medwives were pressuring me HARD to not go more than a week over with my second son. I'd had a week of off-and-on prodromal labor that was driving me batty (get all excited, get all disappointed). So, on Saturday morning, 3 days past my date, I had half a dose. With what I called my "colon bomb breakfast" because of it's immediate effect (handy in case of constipation, LOL). Coffee with cream and a BIG bowl of soaked oatmeal with the works- butter, raisins, nuts, coconut shreds, probably some cream as well. All that fiber, fat, and coffee... yeah...
Well, from what I'd read about castor oil, it works two ways and no one is entirely sure which one is the magic one. First, yes, it irritates your bowels and thereby your uterus. Second, it has some prostaglandins that might cross through the intestines and get to your uterus. All overdue mamas know that prostaglandins are their friends. From hubby or Evening Primrose or castor.
So, I made a shake of sorts with a banana, probably some milk, I'm not sure... whatever I could think of to hide the castor oil. Half a dose, whatever that is (maybe one tablespoon?) went in, and I drank it down.
I figured that the irritation of sorts from my breakfast scrubbing my insides, plus the castor oil, would kick-start things. Or not. But either way, I figured that I wouldn't end up TOO miserable. And I was right.
Within an hour my contractions started. Every 5 minutes (or less), but completely manageable. They lasted all day, getting as close as 3 minutes, until I ate a big dinner and halted things accidentally. I went to bed cranky. And was awoken at 2 am with a HUGE contraction, my water breaking, and real, hard, fast labor. My son was born 4 hours later.
Was it the castor oil? I don't know. I think it was. I had already been taking EPO to make sure my cervix was ready. At my last medwife appt they had said my cervix was lookin good... I don't remember the specifics, but I was maybe 3-4 cm, somewhat effaced, and really soft.
I'll gladly do it again this time, if I need to. I grow incredibly big babies, and so far that has caused them some pretty big problems. I don't want to go more than a few days overdue. But that's MY personal situation.