Castor Oil.

"Castor oil is something that has traditionally been used in a lot of cultures to aid in hair softening and hair growth, but there are no scientific studies that show that it actually improves hair growth or eyelash growth. As an oil it can make a nice coating on the hair that can maybe improve the luster of the hair a little bit, and it might be able to soften brittle eyelashes a bit, but it's still never been shown to actually help with those things. There is a small ophthalmology study that shows that castor oil might help with blepharitis, which is inflammation along the lash line, so if you're struggling to grow your eyelashes because you have chronically irritated red itchy eyes, using castor oil might help reduce that inflammation and allow you to grow better lashes, but I feel like that is a very niche use of this oil. As much as TikTok would like you to believe that castor oil is the miracle oil when it comes to hair care, it's just not. When it comes to hair health and helping with hair brittleness, by far and away the most studied and most proven oil is coconut oil."

"I have a vendetta against castor oil and it's that it doesn't do much of anything and people say it does absolutely everything. There's really almost no study backing of why castor oil would do any of those things, and that's why I'm particularly frazzled when it comes to castor oil because it very much overpromises. There's a cult following, it's kind of a hand me-down ingredient which is fine, that's respectable, there's nothing wrong with it, but at this point there's no reason to think it does much of anything. And there's some thought that it is kind of allergenic. But you could find a better oil for almost any purpose."