Castor oil: what we actually know

Sunday evening, you pour an oil as thick as honey onto your parting. It sticks to your fingers, it strings, and it takes two shampoos to get out. Someone told you it was your grandmother’s secret, or you saw it in one of those tips that keep going round. Castor oil has a reputation for growing hair, eyelashes and eyebrows. What do the studies actually say?

Short answer: nothing solid on growth, a little something on shine. And a rare mishap that’s worth knowing about before you start.

What’s in the bottle

Castor oil comes from the seeds of a plant, Ricinus communis². It has an unusual feature: a single fatty acid dominates. Ricinoleic acid makes up about 90% of its fatty acids². Most of the promises rest on it.

You’ll find it everywhere: in pure bottles, serums, shampoos, lipsticks. It also turns up as a base for blends, for example in some ready-made rosemary oils.

Does it make hair grow?

In 2022, a team of dermatologists reviewed everything published on three widely used oils: coconut, castor and argan. They kept 22 articles¹.

Phong et al., systematic review, 2022¹

Their conclusion is clear: no solid evidence that it helps hair grow¹. A 2026 review of castor oil in dermatology points the same way: its hair-care uses haven’t been studied in a way that shows clinical efficacy².

So why don’t some dermatologists object? The authors of the 2022 review say it plainly: many let their patients carry on with their oils because they have no foreseeable side effects, not because they’ve been shown to work¹. That’s also how cosmetics work: they can be sold without proving any effect on hair loss, unlike a medicine (see the difference between a serum and a medicine).

Where the promise comes from

The promise doesn’t come from nowhere. It starts with a molecule found in the scalp, prostaglandin D2, or PGD2. A 2012 study showed that it slows hair growth and is more abundant in scalps affected by androgenetic alopecia, the inherited form of hair loss².

In the lab, ricinoleic acid binds to the enzyme that makes PGD2 and inhibits it². Hence the reasoning: less PGD2, more hair.

Except the lead hasn’t delivered in humans yet. An experimental medicine that blocks this same pathway was tested in men with androgenetic alopecia, and showed no clinical efficacy². The 2026 review acknowledges it: whether acting on PGD2 can really help in this type of hair loss remains uncertain². And between a molecule that inhibits an enzyme in a test tube and an oil spread on the scalp lies everything nobody has measured.

What it really does: shine

The best-described effect concerns the hair shaft, not the root. A 2003 study measured how light reflects off hair tresses: castor oil increased the contrast between strands, which we see as shinier hair². The 2022 review rates this evidence as weak¹.

Keep the distinction in mind: an oil can improve how hair that has already grown looks, without changing how it grows. The authors of a case published in 2017 put it simply: applying an oil, or a blend of oils, isn’t enough on its own to make hair grow³.

The rare mishap: hair felting

In 2017, dermatologists described the case of a healthy 20-year-old woman with long hair³. For the first time, she had applied a mix of coconut oil and castor oil, hoping to make her hair grow, then washed it³.

As she dried it, her hair suddenly tangled into a hard mass, like a bird’s nest. She saw a doctor the next day. Every attempt to untangle it made it curl even more. The doctors judged the felting irreversible and advised cutting off the matted part; her hair lost about half its length³. The shock was such that she needed psychological support³.

The authors put it down to castor oil’s very high viscosity: on long hair, it sticks the fibres together, the warm washing water presses them flat, and the friction twists them into a single mass. They acknowledge that the exact mechanism isn’t known³.

It’s a very rare mishap: the 2026 review cites only one case, this one². But it’s sudden and can’t be undone. The authors consider coconut oil safer than blends of oils³. And whatever the oil, wash without rubbing the lengths, since friction is what the authors blame. It fits with the gentle habits that protect your hair.

Allergy: rare, but possible

Castor oil is generally well tolerated. Contact allergies have been reported, rarely, and mostly with products that contain it: lipsticks, make-up, shampoos, deodorants². According to the 2026 review, ricinoleic acid is the most likely culprit².

If your scalp itches, reddens or flakes after applying it, stop and talk to your doctor or pharmacist.

Judge by photos, not by promises

Since the studies don’t settle it, the only judge that counts is your own scalp. The trouble is that an oil changes how hair looks: shinier, heavier, sometimes flatter. In the mirror, it’s hard to tell shine from growth.

To see a real difference in density, you need truly comparable photos: same light, same distance, clean dry hair, taken well away from the day you applied the oil. Once a week is enough, for several months.

That’s what Calvap does. Your previous photo appears as a see-through overlay so you can line up the same angle, on the parting, the crown or the temples. You note in the journal the day you start castor oil. Week after week, the timeline shows whether your parting is getting narrower, or whether only the shine has changed.