Nothing much, also its a chore and very messy to add to hair line or hair in general.
Best to try it yourself really some people do claim it does work but its anyone's guess if the science behind it works or if it was another tropical solution.
I've been using it topically every few days. It's definitely a mess. I apply it right after my rogaine (the rogaine acts as a vehicle to penetrate) before bed. It's super cheap and may do something for hair without sides. I am very doubtful that anyone would experience cosmetically significant regrowth via castor oil alone.