Castor oil for melasma?

Has anyone used castor oil to treat melasma? I found a message board online where people swear it works (dozens said it worked for them, and maybe two said it made it worse).

I have been to a dermatologist who insists melasma is only caused by UV, not hormones. She prescribed Tretinoin which I’ve been using for about a year with no change. I’ve also tried Differin but that did nothing. I’ve read to avoid hydroquinone because it can cause irritation and/or uneven lightening.

My melasma is pretty light and seems to be the surface kind, but I am extremely fair. Makeup doesn’t cover it at all, no matter what I try, and color correctors are too dark for my skin.

Any advice?