Castor oil is 89 to 92% ricinoleic acid, an unusual fatty acid that has been studied for its effects on inflammation and pain (Nitbani et al., 2022). Most of that research is in animals or cell cultures. Only one controlled human trial exists, and it used castor oil capsules taken by mouth rather than oil on the skin. Here is what each study found and how far the evidence goes.
How ricinoleic acid may work: the substance P research
The best-studied explanation involves substance P, a messenger released by sensory nerves that carries pain signals and promotes inflammation. Capsaicin, the hot compound in chilli, works partly by depleting it. Ricinoleic acid appears to act in a similar way, without the burning.
- Swelling. In mice, a single topical dose of ricinoleic acid briefly increased inflammatory paw swelling, as capsaicin does. After eight days of repeated application, the same dose markedly reduced swelling, and tissue substance P levels fell. In a longer-running arthritis model, one to three weeks of treatment also reduced swelling, most clearly when it was injected into the skin rather than applied on top (Vieira et al., 2000a).
- Pain sensitivity. In a companion study, eight days of repeated ricinoleic acid reduced pain responses in several mouse models. In an arthritis model, its effect lasted longer than capsaicin’s. Unlike capsaicin, it caused no stinging, irritation or short-term increase in pain sensitivity (Vieira et al., 2000b).
Two points stand out. The effect built up over about a week of repeated use, and an initial single application made swelling worse before it improved. These were animal studies, though, so they show a plausible mechanism rather than an effect in people.
Can ricinoleic acid get through the skin?
A 2015 study tested ricinoleic acid as the oil base of a gel designed to carry medicines through the skin. In human joint-lining cells grown in the lab, the plain ricinoleic acid gel showed more anti-inflammatory activity than a standard gel base. In rats, an anti-inflammatory drug (ketoprofen) worked better at reducing pain and swelling when delivered in the ricinoleic acid gel than in the standard gel (Boddu et al., 2015).
This suggests ricinoleic acid can help carry active ingredients through skin and may add an effect of its own. It doesn’t tell us how much plain castor oil crosses intact human skin.
The one human trial
In a randomised, double-blind trial, 100 people with knee osteoarthritis took either castor oil capsules (0.9 ml, three times a day) or the anti-inflammatory drug diclofenac (50 mg, three times a day) for four weeks. Both groups reported significant symptom relief. Side effects were more common with diclofenac, and none were reported with castor oil (Medhi et al., 2009).
This is encouraging, but it is a single trial, it used castor oil taken by mouth, and it hasn’t been repeated. Castor oil taken orally is a strong laxative, so it shouldn’t be taken this way without professional supervision.
How strong is the evidence?
| What was studied | Type of evidence | What it shows |
|---|---|---|
| Ricinoleic acid on skin, swelling and substance P | Animal studies (mice, guinea pigs) | Less swelling and pain after about 8 days of repeated use |
| Ricinoleic acid as a gel base | Cell and rat studies | May improve delivery of anti-inflammatory ingredients through skin |
| Castor oil capsules for knee osteoarthritis | One human randomised trial, 100 people | Symptom relief comparable to diclofenac over 4 weeks |
| Castor oil packs on skin for joint or muscle pain | No human trials | Unknown |
What we still don’t know
- Whether castor oil applied to the skin reduces inflammation or pain in people. No human trial has tested this.
- How much ricinoleic acid crosses intact human skin from plain castor oil.
- Whether the knee osteoarthritis result holds up in larger trials.
Castor oil is not approved by the TGA to treat inflammation, arthritis or pain. If you have ongoing pain or swelling, see your health practitioner. For how people traditionally use castor oil packs, see our step-by-step castor oil pack guide.
References
- Nitbani FO, Tjitda PJP, Wogo HE, Detha AIR. Preparation of ricinoleic acid from castor oil: a review. J Oleo Sci. 2022;71(6):781–793. doi:10.5650/jos.ess21226
- Vieira C, Evangelista S, Cirillo R, Lippi A, Maggi CA, Manzini S. Effect of ricinoleic acid in acute and subchronic experimental models of inflammation. Mediators Inflamm. 2000;9(5):223–228. doi:10.1080/09629350020025737
- Vieira C, Evangelista S, Cirillo R, Terracciano R, Lippi A, Maggi CA, Manzini S. Antinociceptive activity of ricinoleic acid, a capsaicin-like compound devoid of pungent properties. Eur J Pharmacol. 2000;407(1–2):109–116. doi:10.1016/S0014-2999(00)00727-5
- Boddu SHS, Alsaab H, Umar S, Bonam SP, Gupta H, Ahmed S. Anti-inflammatory effects of a novel ricinoleic acid poloxamer gel system for transdermal delivery. Int J Pharm. 2015;479(1):207–211. doi:10.1016/j.ijpharm.2014.12.051
- Medhi B, Kishore K, Singh U, Seth SD. Comparative clinical trial of castor oil and diclofenac sodium in patients with osteoarthritis. Phytother Res. 2009;23(10):1469–1473. doi:10.1002/ptr.2804