Quick answer: there's no good-quality evidence that castor oil makes hair grow on people. A 2026 review of castor oil in dermatology found no human randomised controlled trials of castor oil for hair growth; the main theory comes from a lab study and hasn't been proven in people. What castor oil can do is coat and soften the lengths, add shine and help reduce breakage, which can help you keep the length you grow. It's thick and sticky, it can irritate some skin, and in very rare cases it has been linked to severe matting of the hair.
Castor oil is one of social media's favourite "growth" hacks, alongside rosemary oil. Here's what is and isn't known.
What is castor oil?
Castor oil is pressed from the seeds of the castor plant (Ricinus communis). Around 90% of its fatty acids are ricinoleic acid, which is unusual among plant oils and gives castor oil its very thick, sticky texture. It has long been used in cosmetics, including lipsticks and hair products, mainly for its texture and shine.
Does castor oil make hair grow?
What the research actually shows
- No human trials: a 2026 narrative review in Cureus (Girdler and colleagues) found no human randomised controlled trials of castor oil for hair growth.
- A lab-based theory: a chemical called prostaglandin D2 is linked to hair thinning in male-pattern hair loss. A preclinical study found ricinoleic acid can block the enzyme that makes it. The review says whether this works in people "remains uncertain".
- The rest is tradition and anecdote: many hair claims come from cultural practice rather than clinical study. That doesn't mean people's experiences aren't real, just that they haven't been tested.
Why people feel it works
Castor oil coats the hair and makes it look glossier and feel thicker. It also adds a protective layer that can reduce friction and breakage on the lengths. Less breakage means you keep more of the length you grow, which can look like faster growth even though the hair isn't growing any faster from the root.
What castor oil can and can't do
| Can | Can't (on current evidence) |
|---|---|
| Coat and soften dry lengths and ends | Make hair grow faster from the root |
| Add shine | Regrow hair or treat hair loss |
| Help reduce friction and breakage | Thicken individual strands permanently |
| Seal very dry, coarse or textured ends | Repair damaged hair permanently |
Jamaican black castor oil: is it better?
Jamaican black castor oil is made by roasting the seeds before pressing, which gives it a darker colour and a smoky smell. It's especially popular in textured hair care. There's no clinical evidence that it grows hair better than regular castor oil, or that either one grows hair at all.
Castor oil for eyelashes and eyebrows
This is one of the most popular uses, but there's no good evidence that castor oil lengthens lashes or regrows brows. Getting oil in your eyes can cause irritation, so be very careful around the eye area. If your brows or lashes are thinning, see your GP or optician rather than relying on an oil.
Side effects and safety
- Skin irritation: the review notes contact dermatitis is the most common side effect, usually mild. Patch test on a small area first, and stop if your skin goes red, itchy or sore.
- Hair matting: very rarely, castor oil has been linked to "acute hair felting", where hair suddenly tangles into a solid mass. In the case described, the hair had to be cut off. It's extremely rare, but worth knowing about, especially on long hair.
- It's hard to wash out: its thickness means it can leave hair greasy and heavy, and may need two washes or a clarifying shampoo to remove.
- Not on a flaky scalp: oil on the scalp can make dandruff worse for some people. See dandruff treatment.
If you want to try it on your hair
- Patch test first.
- Mix a small amount with a lighter oil so it spreads more easily.
- Apply to the mid-lengths and ends as a pre-wash treatment, not as a leave-in.
- Leave for 30 minutes to an hour, then shampoo, twice if needed.
- Detangle gently with conditioner in, and don't pile long hair on top of your head while the oil is on.
Lighter alternatives for shine and softness
If what you're after is softer, shinier, less breakable lengths, a lighter oil does that without the stickiness:
- Illuminating Hair Oil-Serum: rice bran, sweet almond and argan oils with organic coconut, chia and sacha inchi, especially for waves, curls and coils.
- Bouncy Curls Hair Oil, winner of Best Afro Hair Oil 2023, for curls and textured hair.
- Oil Non Oil for fine hair that hates weight.
More in our guides to hair oil and the best hair oil for curly hair.
Worried about hair loss?
If you're losing more hair than usual, have patches of thinning or a change you can't explain, speak to your GP. Hair loss has many causes, some of which need proper diagnosis and treatment. You may also find our guides to seasonal hair shedding and menopause hair changes helpful.
Frequently asked questions
Does castor oil thicken hair?
It can make hair look and feel thicker because it coats each strand, but there's no good evidence it increases the number of hairs.
Can I leave castor oil in my hair overnight?
Some people do, but it's very heavy, can stain bedding and is hard to wash out. A shorter pre-wash treatment is usually enough.
Is castor oil or rosemary oil better for hair growth?
Neither has strong evidence. Rosemary oil has a little more research behind it, but even that is limited. Read what the evidence says about rosemary oil.
Does Insight make a castor oil hair treatment?
No. Our hair oils focus on shine, softness and frizz control. If your hair is fine and fragile, the Fortifying Hair Treatment is a no-rinse lotion that strengthens weak, fragile hair and leaves the scalp feeling fresh.
Source: Girdler K, Cabatu A, Olds H, Potts GA. "Use of Castor Oil in Dermatology: A Narrative Review." Cureus, 2026; 18(2): e103289.
About the author
Charlotte Read has spent six years in the hair industry with Insight Professional and holds a First Class BSc in Business Management and Marketing. She writes with reference to real salons across social media and the wider Insight community, and draws on the Insight office team, who between them bring over 100 years of combined experience on salon floors.
Where technical expertise is needed, Charlotte consults Lorraine Matic, Head of Insight UK Education, who brings over 38 years of global experience in colour, technique and hair science.