Castor oil

What it is

Castor oil is pressed from the seeds of Ricinus communis. Most of its fatty acid content is ricinoleic acid, which makes it unusually viscous. Jamaican black castor oil is made from roasted seeds, giving a darker, more alkaline product. Sellers often claim the black kind is better for growth, but no study compares the two for hair.

How it works

• As a cosmetic it coats the shaft and seals in moisture, adding shine and reducing friction.
• A 2026 narrative review proposes that ricinoleic acid may inhibit prostaglandin D2 synthesis, since PGD2 is raised in balding scalp. This is a hypothesis, and no hair-loss trial tests it (Girdler 2026).

What the studies show

• A systematic review of 22 articles on coconut, castor and argan oils found weaker evidence that castor oil improves luster and no strong evidence for growth. Phong 2022
• A review of hair oils used in the Black community noted that most data on castor oil for pattern hair loss come from mice. Dinkins 2023
• A systematic review of 8 studies found no clinical trials of natural ingredients, including black castor oil, for central centrifugal cicatricial alopecia. Ezekwe 2020
• A 2026 review supports luster, calls a growth benefit "possible", and lists contact dermatitis and hair felting as rare adverse effects. Girdler 2026

The growth claim rests on reviews and extrapolation, not on a human trial of castor oil alone.

How it's used in studies

No dose-finding exists. In everyday practice it is massaged into the scalp or lengths, often diluted with a lighter oil, and left for hours or overnight before shampooing.

Side effects and safety

• Allergic or irritant contact dermatitis is rare (Girdler 2026).
• Acute hair felting: one published case describes the whole scalp matting after coconut and castor oil were applied to long hair after a wash. The hair could not be untangled and had to be cut off (Maduri 2017). It is one case, so the risk is unquantified, but the consequence is severe.
• Heavy oil can trap sebum and debris, so people with seborrheic dermatitis or folliculitis-prone scalps should be cautious.
• Do not swallow it as a hair treatment. It is a stimulant laxative and is used to induce labour in some settings, so avoid it in pregnancy.

Bottom line

Castor oil is a cheap sealant for dry, coarse, high-porosity or coily hair that is clean and detangled. Do not expect regrowth. Dilute it, apply it to combed hair, and take care on long, tangle-prone hair that is washed rarely. Growth evidence is weak (hypothesis plus anecdote); the benefit for softness and shine is weak but plausible. Hair loss that matters needs a proven treatment (see the hair loss treatment ladder).

Fun facts

The prostaglandin D2 idea echoes a real drug pathway: PGD2-receptor blockers such as setipiprant have been tested for pattern baldness. Whether castor oil does anything similar on the scalp has not been tested.