Thoughts on castor oil?

I am 39 weeks 4 days pregnant. At my last appointment last tuesday I was 1cm 50% effaced. For the last few days I've been having bad back pains that I'm pretty sure are due to constipation. I want this baby OUT and I'm seriously considering using castor oil to help with the constipation and possibly start my labor. I was wondering what experiences others have had with this so please feel free to share and thank you in advance! (By the way I only started really thinking about it after an advice nurse on the kaiser hotline suggested it to help)