Castor Oil and Olive Oil: How They Differ, and When to Mix Them
Castor oil and olive oil sit side by side in a lot of bathroom cabinets, and the internet is full of recipes that mix them for hair, brows and skin. What most of those recipes skip is the plain chemistry: the two oils are built from different fatty acids, they behave very differently on hair and skin, and only one of them is a food. Castor oil is up to 90% ricinoleic acid, an unusual fatty acid that makes it thick, sticky and, when swallowed, a stimulant laxative. Olive oil is dominated by oleic acid, a common fatty acid that makes it a lighter, more spreadable oil that you can also cook with. This guide sets out how they differ, what the evidence says each one does, when mixing them makes sense, and the safety points that the recipes tend to leave out.
For our own oil, see our Cold-Pressed Single-Estate Extra Virgin Olive Oil — Marrakech (500ml).
The Short Answer
- Castor oil, pressed from the seeds of Ricinus communis, is up to 90% ricinoleic acid, with only around 4% linoleic and 3% oleic acid (Patel VR et al., Lipid Insights 2016, PMID 27656091).
- Olive oil is mostly oleic acid: the International Olive Council's trade standard sets oleic acid at 55% to 85% of the fatty acids in olive oils, with linoleic acid at 2.5% to 21%.
- That difference explains the feel. Ricinoleic acid carries an extra hydroxyl group, which makes castor oil far thicker and stickier than olive oil, so it is usually diluted rather than used neat on long hair.
- Neither oil has good evidence for growing hair, brows or lashes. A 2022 systematic review of 22 articles found only weak evidence that castor oil improves hair lustre and no strong evidence that it helps hair grow.
- Mixing them is reasonable for conditioning hair and grooming brows: olive oil thins castor's texture. Many people start at about one part castor to two or three parts olive oil.
- Castor oil is not a cooking oil. Taken by mouth it is a stimulant laxative, and it should never be used in pregnancy or to try to bring on labour.
Castor Oil vs Olive Oil: The Composition Difference
Every plant oil is mostly triglycerides, and what sets one oil apart is which fatty acids those triglycerides carry. Here the two oils could hardly be further apart. A review by Patel VR and colleagues (Lipid Insights 2016;9:1–12, PMID 27656091) describes castor oil as up to 90% ricinoleic acid, a fatty acid that is rare in other common oils. The same review notes that castor oil's viscosity is higher than that of a standard SAE 40 engine oil, which gives some sense of how thick it is.
Ricinoleic acid is an eighteen-carbon fatty acid with a hydroxyl (OH) group partway along its chain. That small chemical difference makes castor oil more polar and much more viscous than ordinary vegetable oils. Olive oil, by contrast, is built mainly from oleic acid, the same monounsaturated fatty acid found in avocado and many nuts, and it pours freely at room temperature.
| Castor oil | Olive oil (extra virgin) | |
|---|---|---|
| Source | Seeds of Ricinus communis | Fruit of the olive tree, Olea europaea |
| Main fatty acid | Ricinoleic acid, up to 90% | Oleic acid, 55% to 85% |
| Texture | Very thick, sticky, slow to spread | Fluid, spreads easily |
| Colour | Pale, near-colourless to straw | Golden to green-gold when extra virgin |
| Taken by mouth | A stimulant laxative, not a food | An everyday food and cooking fat |
| Typical topical use | Hair and scalp oils, brows and lashes, lip products | Hair conditioning, massage, skin and beard oils |
Extra virgin olive oil also carries polyphenols and other minor compounds, which matter mainly when it is eaten; our guide to choosing a quality olive oil explains what separates extra virgin oil from refined grades.
What Each Oil Does for Hair, and What Neither Does
On hair, both oils work as conditioners. They coat the strand, reduce friction between hairs and make combing easier, which in turn means less breakage. Olive oil is the lighter of the two and spreads through the lengths easily; our article on olive oil for hair covers how to use it as a pre-wash treatment. Castor oil's weight gives a smoother, glossier finish but is harder to wash out.
The claim that castor oil grows hair is where the evidence runs out. Phong C and colleagues (Journal of Drugs in Dermatology 2022;21(7):751–757, PMID 35816075) reviewed 22 articles on coconut, castor and argan oil for hair. They found weaker evidence that castor oil improves hair quality by increasing lustre, and no strong evidence supporting it for hair growth. We found no human trial showing that olive oil, or a mix of the two, regrows hair. If you are losing hair, a GP or dermatologist is the right place to start, because thinning has many causes that an oil cannot address.
Because recipes often add coconut oil to the mix, one study is worth knowing. Rele AS and Mohile RB (Journal of Cosmetic Science 2003;54(2):175–192, PMID 12715094) found that coconut oil reduced protein loss from hair, while mineral oil and sunflower oil did not, which is why coconut is often used as a pre-wash treatment alongside the other two.
Skin, Face, Brows and Lashes
On skin, the honest picture is more cautious than most recipes suggest. Danby SG and colleagues (Pediatric Dermatology 2013;30(1):42–50, PMID 22995032) asked 19 adult volunteers to apply six drops of olive oil to a forearm twice a day. After four weeks, olive oil had significantly reduced the integrity of the skin's outer layer and caused mild redness, while sunflower seed oil did not. That was a small forearm study, but it is a reason not to use olive oil as a daily leave-on moisturiser for dry or eczema-prone skin; our guide to olive oil for skin covers the wider picture.
Castor oil has its own skin issue: it is a recognised, if uncommon, cause of allergic contact dermatitis. Published case reports include allergic inflammation of the lips from a castor oil lip balm (Sánchez-Herrero A et al., Contact Dermatitis 2018;79(3):176–178, PMID 29749619). Popular "oil cleansing" mixes of castor and olive oil for the face have not been tested in trials that we could find.
For brows and lashes, a little oil can make hairs look glossier and lie flatter, which is the main reason people use it. Neither oil has trial evidence for making brows or lashes grow. Keep both oils out of the eye itself: apply a small amount to the brow with a clean spoolie, or to the base of the lashes with a clean brush, and stop if your eyes sting or become red. See our guides to olive oil for eyebrows and olive oil for eyelashes for more detail.
When to Mix Castor Oil and Olive Oil, and How
Mixing the two makes most sense when you want castor oil's gloss and weight without its stickiness, for example as a pre-wash hair treatment or a brow oil. Olive oil thins the blend so it spreads further and rinses out more easily. There is no tested "correct" ratio; the figures below are practical starting points, not results from a study.
- Start with one part castor oil to two or three parts olive oil, mixed in a small, clean dark glass bottle.
- For fine or oily hair, go lighter on castor; for thick, coarse or very dry hair, you can move towards equal parts.
- Patch test the blend first: apply a little to the inside of your forearm or behind your ear and wait at least 24 hours for redness, itching or swelling.
- For hair, work it through the mid-lengths and ends, leave for 30 minutes to a few hours, then shampoo thoroughly; you may need two washes.
- For brows, apply a single drop with a clean spoolie at night, away from the eyes.
- Mix small batches, keep the bottle closed and away from heat and light, and throw it away if it smells rancid.
One rare risk applies to long hair. Maduri VR and colleagues (International Journal of Trichology 2017;9(3):116–118, PMID 28932063) reported a case of sudden, irreversible matting of the whole scalp, called hair felting, after coconut and castor oil were applied following a wash; the authors linked it to castor oil's high viscosity and long hair, and the hair had to be cut off. It is uncommon, but it is a good reason to dilute castor oil and avoid piling long hair on top of the head while it is oiled. Black seed oil blends are a separate topic, covered in our article on black seed castor oil.
Castor Oil Is Not a Food Oil
This is the most important difference between the two. Olive oil is a food. Castor oil, swallowed, is a drug. In the United States it is sold over the counter as a stimulant laxative, and its Drug Facts labelling warns against use for longer than a week and tells anyone pregnant or breastfeeding to ask a health professional first. The UK's NHS guidance on laxatives names senna and bisacodyl as its stimulant laxatives and does not include castor oil.
The mechanism was clarified by Tunaru S and colleagues (Proceedings of the National Academy of Sciences 2012;109(23):9179–9184, PMID 22615395). In mice, ricinoleic acid activated EP3 prostanoid receptors on the smooth muscle of the gut and the uterus, producing both the laxative effect and uterine contractions. That second effect is why castor oil should never be used in pregnancy or to try to bring on labour. The NHS states that there are no proven ways of starting labour yourself at home; anything to do with induction belongs with your midwife or doctor.
So do not add castor oil to food, salad dressings or "detox" drinks, and do not take it by mouth unless a doctor or pharmacist has advised it. The seeds themselves should never be eaten either: Patel and colleagues note that castor seeds contain ricin, a potent plant toxin. If constipation is the problem, a pharmacist can recommend a suitable laxative.
The Objections Worth Taking Seriously
"Castor oil is so thick that it must be doing something to the follicles." Thickness is a physical property, not evidence of growth. Castor oil's viscosity explains why it feels rich and coats hair well; the 2022 systematic review found no strong evidence that it makes hair grow.
"People have used these oils on their hair for generations." They have, and the conditioning benefit people notice is real. Tradition tells you an oil is pleasant and generally well tolerated on hair; it does not tell you it regrows hair, and with castor oil taken by mouth, the traditional use is exactly the reason for caution.
"You sell olive oil, so of course you prefer it." We do sell olive oil and we do not sell castor oil, so read us with that in mind. But our point is not that olive oil is better; the two oils do different jobs, and on skin the best controlled study we found was unflattering to olive oil, which we have reported above.
Why Sidr & Stone
If you are mixing olive oil into a hair or brow oil, or keeping a bottle in the kitchen that also does duty on your hair, the quality of that oil still matters: fresh, cold-pressed extra virgin olive oil has not been refined. Sidr & Stone's Cold-Pressed Single-Estate Extra Virgin Olive Oil comes from our own family-owned farm on the plains outside Marrakech, where we control every step from tree to bottle.
- Single-estate — our own family-owned farm near Marrakech, Morocco; no blending across origins
- Rain-fed — no irrigation; the trees take what the season gives
- Organically grown — no synthetic fertilisers, pesticides or herbicides (not yet certified; certification in progress)
- Single harvest — a small, limited batch; once it is gone, it is gone until the next harvest
- Cold-pressed within hours of harvest — to protect flavour, aroma and polyphenols
- Unfiltered extra virgin — minimally processed; natural sediment is normal
- 100% natural — a single ingredient, nothing added
- Dark glass with a gold label — 500ml, packaged to protect the oil from light
- 10% of profits to charity
- Fulfilment in the UK, EU, and US
We will not tell you Sidr & Stone is the best olive oil for your hair — that would be the very kind of claim this article warns against. What we will say is that our oil is single-estate Moroccan, rain-fed, organically grown and cold-pressed within hours of harvest, and that the evidence of that care is in the taste, the colour and the small single-harvest batch.
Frequently Asked Questions
Can you mix castor oil and olive oil?
Yes, for use on hair, brows and skin. Olive oil thins castor oil's sticky texture, so the blend spreads and washes out more easily. Many people start at one part castor oil to two or three parts olive oil.
What is the difference between castor oil and olive oil?
Castor oil is up to 90% ricinoleic acid, which makes it very thick and, when swallowed, a stimulant laxative. Olive oil is mainly oleic acid (55% to 85% under the International Olive Council's standard), so it is lighter and is eaten as an everyday food.
Do I need to patch test a castor and olive oil blend?
Yes. Apply a little of the blend to the inside of your forearm or behind your ear and wait at least 24 hours before using it on your scalp, face or brows. Castor oil is an uncommon but recognised cause of allergic contact dermatitis.
Is castor oil or olive oil better for eyebrows?
Neither has trial evidence for growing brows. Castor oil gives more gloss and hold, while olive oil is lighter; a drop of a blend applied with a clean spoolie, away from the eyes, gives you some of both.
How is Sidr & Stone's olive oil different from supermarket olive oil?
It is single-estate, rain-fed, organically grown and cold-pressed within hours of harvest on our own farm outside Marrakech, and it is unfiltered extra virgin. Many supermarket oils are blends from several countries, and some are refined.
Can I drink castor oil mixed with olive oil?
No. Castor oil taken by mouth is a stimulant laxative, and mixing it with olive oil does not change that. Do not take it unless a doctor or pharmacist has advised it, and never use it in pregnancy or to try to bring on labour.
Where can I buy Sidr & Stone olive oil?
Our Cold-Pressed Single-Estate Extra Virgin Olive Oil — Marrakech (500ml) is available to pre-order as a limited single-harvest batch directly from our website, with fulfilment in the UK, EU, and US; the product page shows current shipping dates.
Is olive oil a medicine?
No. Olive oil is a food, not a medicine. It has a long traditional history — including being honoured in the Prophetic Sunnah — and a substantial body of modern research, particularly around polyphenols, cardiovascular health, and the Mediterranean diet pattern. It can be a worthwhile part of a healthy routine, but it does not cure diseases and is not a substitute for medical care. Be cautious of any olive oil marketed with specific disease-cure claims.
Final Thoughts
Castor oil and olive oil are both useful, but they are not interchangeable. Castor oil is thick, sticky and almost entirely ricinoleic acid, which makes it a heavy conditioner on hair and a laxative in the body. Olive oil is lighter, built from oleic acid, and belongs in the kitchen as much as the bathroom.
Mixed together, they make a sensible hair or brow oil, as long as you patch test first, dilute the castor oil, wash it out properly and keep it out of your eyes. What the blend will not do is grow hair, and what castor oil should never do is go into your food, your drinks or a pregnancy.
Our Cold-Pressed Single-Estate Extra Virgin Olive Oil — Marrakech (500ml) — single-estate, rain-fed and cold-pressed within hours of harvest — is available to pre-order now, with fulfilment in the UK, EU, and US.
Our extra virgin olive oil is grown organically on our own single-estate farm on the plains outside Marrakech: we control every step from tree to bottle, and no synthetic pesticides, herbicides or fertilisers are used. It is not yet certified organic — certification is in progress, and we will publish it as soon as it comes through.
Disclaimer: This article describes the composition of castor oil and olive oil and published research on their use at the time of writing; research findings and product guidance may change, and readers should check current sources. Olive oil is a food, not a medicine, and is not a substitute for medical treatment of any condition. Castor oil should not be taken by mouth except on the advice of a doctor or pharmacist, and should not be used in pregnancy. Patch test any oil before applying it to your skin, scalp or brows. For any health concern, consult a qualified medical professional.