Coconut oil is the only one of the three with solid clinical backing—it actually works for brittle hair and head lice. Castor oil has some weak evidence for making hair shinier, but nothing proven for growth. Argan oil? No meaningful evidence supports it for hair growth, quality, or lice treatment.

Researchers reviewed 22 published studies comparing these three oils for hair and scalp health. They looked specifically at whether the oils could improve hair growth, quality, and treat infestation.

Funding not disclosed in abstract