Castor Oil for Hair: Does It Actually Grow Hair?
INCI: Ricinus Communis Seed Oil · also listed as Castor oil, Ricinus communis (castor) seed oil, Jamaican black castor oilThe most-searched hair-growth oil on the internet, and one of the oldest. Safe, cheap and genuinely useful on coarse, dry hair, and never shown to grow a single extra hair.
A good sealant, not a growth treatment
Castor oil is about 90% ricinoleic acid, an unusual fatty acid that makes the oil thick, sticky and clinging. On coarse, high-porosity or coily hair that is what a sealant should be: it slows water loss and lays down frizz. On fine or wavy hair it is a weight the strand cannot carry.
The growth claim has no clinical trial behind it. Its best support is a 2015 computer model, and a drug built on the same pathway later failed its trial. Mixed for fit, not risk, with one rare exception below.
- INCI name
- Ricinus Communis Seed Oil
- Also on labels
- Castor oil. Jamaican black castor oil is the same oil from roasted beans: darker, slightly alkaline. Not PEG-40 hydrogenated castor oil, which is an emulsifier.
- Job in hair care
- Occlusive emollient. Coats the shaft to slow water loss, smooth lifted cuticle and add weight and shine. Does not penetrate like coconut oil.
- Found in
- Hair oils and serums, pre-shampoo treatments, edge and scalp balms, and sold neat. Rare in shampoos.
- Typical level
- 1–10% in a finished product, usually cut with lighter oils. Neat use is where the problems start.
- Regulatory status
- Permitted worldwide, no cosmetic limit. Rated safe by the US Cosmetic Ingredient Review panel. Not a licensed hair-loss treatment anywhere.
- Hairmony class
- ActiveScored by the Hairmony engine, so it actually moves which products you get recommended.
- Scored for
- +moisture+frizz control+porosity repair−fine or wavy hair−over-moisturised hairScored as a heavy sealing oil: moisture, frizz and cuticle-smoothing positives, weighted down on fine hair. Its hair-growth weight is deliberately small.
What castor oil does to hair and scalp
On the hair shaft it is a sealant. Roughly ten times thicker than olive oil, it stays where you put it, fills a rough cuticle and gives coarse or coily hair a heavy, glossy softness. It does not penetrate like coconut oil; it protects from the outside only.
On the scalp it is well tolerated and mildly antimicrobial, and massaging it in feels productive. The massage may be; the oil has not been shown to be. The study everyone cites was a computer model, and a drug blocking the same pathway did nothing for hair growth in 169 men.
Worth using if
- Your hair is coarse, thick, high-porosity or coily
- You seal with an oil after a leave-in (the "O" in LOC)
- Your ends are dry and fraying between trims
Be careful if
- Your hair is fine, wavy or goes flat easily
- You have long hair and apply it neat (see below)
- You expect it to regrow a thinning hairline
In practice: cut it 1:3 with a lighter oil, apply to the ends only, and clarify with a sulfate shampoo every couple of weeks. Nothing milder shifts it.
Is castor oil harmful? The one real risk
Toxicity is not the issue: the beans are poisonous, the pressed oil is not. The one documented harm is mechanical. Acute hair felting is a sudden, irreversible matting of the hair into one solid mass, and castor oil is the usual trigger in case reports: its viscosity lets long hair lock during a wash.
It is rare and needs a combination: long hair, a heavy neat application, vigorous washing. But the only treatment is cutting the mass out. On long hair, dilute it, keep it off the mid-lengths, and comb through before you wet it.
What we based this on
- Hair felting. Maduri et al., "Castor Oil": The Culprit of Acute Hair Felting, Int J Trichology 2017: a first neat application on long hair, matted within a day, cut off.
- The growth claim. Fong et al. 2015, an in-silico prediction that ricinoleic acid may inhibit prostaglandin D2 synthase. A model, not a human study. A drug on that pathway, setipiprant, then showed no benefit over placebo in a 2021 trial.
Not medical advice. This page describes how an ingredient behaves on hair and scalp. If you have a persistent rash, sores, sudden shedding or a diagnosed scalp condition, see a dermatologist before changing anything.
Frequently asked
Does castor oil make hair grow faster?
No human study has shown that. The claim rests on a 2015 computer model about ricinoleic acid and a hair-loss enzyme, and a drug targeting that same pathway failed its clinical trial. Castor oil can reduce breakage on dry ends, which keeps length, but it does not change growth rate.
Is castor oil good for a dry scalp?
It can soothe a dry, tight scalp short term because it is occlusive and mildly antimicrobial. It also does not rinse out easily, so on an oily or flaky scalp it tends to trap sebum and scale. Use a few diluted drops, not a coat, and clarify afterwards.
Can I leave castor oil in my hair overnight?
On short or medium hair, diluted, yes. On long hair applied neat, avoid it: heavy castor oil plus a vigorous wash is the classic setup for acute hair felting, a rare matting that can only be cut out. Dilute it, keep it to the ends and comb before wetting.
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