Castor oil is a strong conditioner rather than a proven growth treatment: clinical trials have not yet tested castor oil on its own for hair growth. What it does well is coat and condition the hair shaft, adding shine and slip and cutting down the friction that leads to breakage, so hair can look and feel thicker. It works best as part of a scalp and hair routine, often blended with lighter oils such as rosemary, jojoba or argan.
Key takeaways
- Castor oil is made up of up to about 90% ricinoleic acid, a fatty acid that gives it its thick, sticky texture.
- A 2022 systematic review found some evidence that castor oil improves hair luster (shine); hair-growth claims for castor oil alone have not been tested in clinical trials.
- The popular "castor oil blocks PGD2" idea comes from a 2015 computer-modeling study that flagged ricinoleic acid as a candidate for further lab testing.
- Castor oil is very thick, so diluting it with a lighter oil and washing it out well makes it easier to use.
- For thinning hair, ingredients such as rosemary and caffeine have their own clinical studies and pair well with castor oil in a routine.
What is castor oil?
Castor oil is pressed from the seeds of the castor plant, Ricinus communis. Its standout feature is its fatty acid profile: according to a review by Patel and colleagues (2016), it contains up to 90% ricinoleic acid, with small amounts of linoleic, oleic and stearic acids.
Ricinoleic acid makes castor oil far thicker and stickier than most plant oils. That texture is why it clings to the hair shaft, why a little goes a long way, and why it takes a thorough wash to remove. Jamaican black castor oil comes from roasted seeds and is darker, but its fatty acid makeup is similar.
What clinical studies have tested
Research on castor oil for hair has focused on hair quality, with growth trials still to come. A 2022 systematic review by Phong and colleagues looked at 22 studies on coconut, castor and argan oils for hair. It found some evidence that castor oil improves hair luster, and no strong evidence for hair growth.
That puts castor oil in a different place from some other popular scalp ingredients. Rosemary has a randomized trial against minoxidil behind it, which we cover in our rosemary oil research review. Caffeine has several clinical studies, which we go through in our article on caffeine for hair growth. Castor oil's research strengths are shine and conditioning.
Where does the PGD2 theory come from?
It traces back to two separate studies: a solid discovery about a hormone-like molecule in balding scalp, and a later computer prediction linking a castor oil component to it.
In 2012, Garza and colleagues found that the enzyme prostaglandin D2 synthase and its product, prostaglandin D2 (PGD2), are elevated in the bald scalp of men with androgenetic alopecia compared with haired scalp. PGD2 slowed hair growth in human hair follicles grown in the lab and when applied to mice, and that effect ran through a receptor called GPR44. The paper pointed to the PGD2 pathway as a potential target for new treatments. It studied PGD2 itself, not castor oil.
The castor oil link came in 2015. Fong and colleagues used computer docking to screen 389 compounds from 12 traditional herbs against six crystal structures of the PGD2-producing enzyme. Ricinoleic acid was one of five compounds predicted to block the enzyme while also having good properties for use on skin. The authors flagged these compounds for the next step: testing in cells and animals.
So the chain is a real hormone, a real receptor, and a promising computer prediction about one of castor oil's main components. Lab and human studies are the next steps that would show whether castor oil applied to the scalp affects PGD2.
What does castor oil do for hair?
It conditions. As a thick emollient, castor oil coats the hair shaft, smooths the cuticle and adds slip between strands.
Less friction means less mechanical breakage from brushing, styling and sleeping on your hair, and fewer broken ends make hair look fuller. The coating also reflects light, which fits the luster finding in the Phong review. These effects show up within days, because they happen along the hair shaft, and they build as fewer strands snap over the following weeks.
On the scalp, castor oil can help a dry, tight scalp feel more comfortable, much like other plant oils. If your flakes are oily and itchy rather than dry, read our guide to itchy, flaky scalp first, because seborrheic dermatitis responds to medicated shampoos rather than oils.
Castor oil claims and the evidence
| Claim | What the evidence shows | Verdict |
|---|---|---|
| Makes hair shinier | Some evidence of improved luster in a 2022 systematic review | Supported |
| Reduces breakage, hair feels thicker | Consistent with how thick emollient oils coat and smooth the shaft | Plausible cosmetic effect |
| Soothes a dry scalp | Moisturizing oils ease dryness and tightness | Reasonable for dry scalp |
| Blocks PGD2, the "baldness hormone" | Computer prediction for ricinoleic acid (2015); lab testing still to come | Early-stage idea |
| Grows new hair | Clinical trials of castor oil alone have not been run yet | Not yet tested |
How do you use castor oil on your hair?
Use a small amount, dilute it, and wash it out well. Castor oil's thickness is its strength and its main challenge.
- Dilute it. Mix one part castor oil with two or three parts of a lighter oil such as jojoba or argan. It spreads more easily and rinses out more cleanly.
- Decide where it goes. For dry, brittle lengths and ends, smooth a few drops through damp or dry hair. For a dry scalp, part the hair and massage a small amount into the skin with your fingertips.
- Leave it on. Thirty minutes to a few hours works well as a pre-wash treatment, or overnight with a towel on your pillow.
- Wash it out. Shampoo thoroughly; you may need two rounds. Leaving heavy oil on the scalp for days can build up.
- Keep long hair loose and smooth. A rare case report described sudden, severe matting ("hair felting") of long hair after washing with coconut and castor oil, which the authors put down to castor oil's high viscosity (Maduri 2017). Detangling gently and not piling oiled long hair on top of the head helps avoid tangles.
- Patch test first. Try a little on the inner arm for a day if you have sensitive skin.
Once or twice a week is plenty for most people. A gentle fingertip massage while you apply it is a pleasant routine in itself; our article on scalp massage for hair growth covers technique.
Where does castor oil fit in a hair routine?
Castor oil works best as a conditioning step alongside ingredients chosen for the scalp. Because it is so thick, most people get on better with it in a blend than on its own.
That is how we use it in the Folliq Rosemary & Batana Hair Oil: black castor seed oil sits alongside batana kernel oil and rosemary leaf oil, so you get its conditioning, shine and smoother feel in a texture that absorbs quickly, feels non-greasy and leaves no build-up. Warm a few drops, massage about 3 drops into each of five scalp areas (front, back, left, right and crown), then smooth what is left through your lengths; it suits a daily scalp routine for all hair types. For thinning, pair a conditioning oil with a scalp routine built around the ingredients and tools that have their own studies, as laid out in our guide on how to build a scalp care routine.
When to see a dermatologist
- Thinning at the crown or temples, or a widening part, rather than dryness or broken ends.
- Shedding that is sudden or heavy, or comes out in patches.
- Scalp redness, itching, pain or flaking that does not settle with a gentler routine.
- A rash, burning or swelling after applying any oil.
A dermatologist can tell breakage from true hair loss and recommend treatments with trial evidence for your type of thinning.
FAQ
Does castor oil make hair thicker?
It can make hair look and feel thicker by coating each strand and reducing breakage. That is a conditioning effect on the hair you have; clinical trials of castor oil for new growth have not been run yet.
How long does it take for castor oil to work on hair?
The conditioning effects, such as shine, softness and smoother strands, show up within the first few uses. Less breakage builds over a few weeks of regular use as fewer strands snap.
Is Jamaican black castor oil better for hair growth?
Jamaican black castor oil is made from roasted seeds and is darker, but its fatty acid makeup is similar to regular castor oil. We found no studies comparing the two for hair, so choose the one whose texture and scent you prefer.
Can I leave castor oil in my hair overnight?
Yes, especially diluted with a lighter oil. Protect your pillow, keep long hair loosely braided, and shampoo it out thoroughly in the morning.
Is castor oil or rosemary oil better for hair growth?
Rosemary has more hair-growth research behind it, including a randomized trial, while castor oil's strength is conditioning and shine. They work well together, which is why many blends combine them.
Folliq products are cosmetic scalp-care products, not medical devices or medicines, and are not intended to diagnose, treat, cure or prevent any disease. The studies discussed here used clinical devices and preparations, which is why every Folliq product comes with 90 days to try it for yourself. If your hair loss is sudden, patchy or painful, see a dermatologist.
References
- Patel VR, Dumancas GG, Kasi Viswanath LC, Maples R, Subong BJ. Castor oil: properties, uses, and optimization of processing parameters in commercial production. Lipid Insights. 2016;9:1–12. PubMed 27656091
- Phong C, Lee V, Yale K, Sung C, Mesinkovska N. Coconut, castor, and argan oil for hair in skin of color patients: a systematic review. Journal of Drugs in Dermatology. 2022;21(7):751–757. PubMed 35816075
- Garza LA, Liu Y, Yang Z, et al. Prostaglandin D2 inhibits hair growth and is elevated in bald scalp of men with androgenetic alopecia. Science Translational Medicine. 2012;4(126):126ra34. PubMed 22440736
- Fong P, Tong HH, Ng KH, Lao CK, Chong CI, Chao CM. In silico prediction of prostaglandin D2 synthase inhibitors from herbal constituents for the treatment of hair loss. Journal of Ethnopharmacology. 2015;175:470–480. PubMed 26456343
- Maduri VR, Vedachalam A, Kiruthika S. "Castor oil" – the culprit of acute hair felting. International Journal of Trichology. 2017;9(3):116–118. PubMed 28932063