Does castor oil cause yellowing of hair?

Castor oil (INCI: Ricinus Communis (Castor) Seed Oil) is a vegetable oil obtained by cold pressing seeds of the castor plant (Ricinus communis), a shrub cultivated in China, India, and Brazil. Castor oil appears as a thick, yellow liquid in organoleptic assessment and is known for its nourishing and strengthening properties, which stem from its saturated fatty acid content. These acids have a chemical structure similar to that of cuticle lipids, allowing them to insert by biomimicry and reinforce the cohesion of this layer. Though studies remain limited, castor oil may stimulate hair growth, a property that contributes to its popularity. This effect may result from its ability to inhibit prostaglandin D2 (PGD2), a protein involved in androgenetic alopecia.

Despite its many benefits for hair, castor oil is criticised for its thick, viscous texture, and alleged yellowing effect. No scientific study has reported or demonstrated this effect.

Only a few reports mention the yellowing effect of castor oil when it is applied in large quantities and left on hair for extended periods. One hypothesis is that this phenomenon depends on the hair’s natural pigmentation, lighter or bleached hair being more prone to staining due to its porosity, porous hair absorbs more of the product applied to its surface, and the oxidation state of hair fibres, oxidised fibres being more fragile. The apparent yellowing observed by some individuals could also be an optical effect. Indeed, the castor oil has a natural yellow tint, when applied to hair, it could appear yellowed if rinsing is inadequate. However, in the absence of scientific studies on the subject, these remain hypotheses.

Although no scientific evidence indicates that castor oil causes hair yellowing, here are some tips to continue benefiting from its application to hair.

• Apply a few drops of castor oil.

  Castor oil is viscous and rich in lipids, so large amounts are unnecessary. It may weigh down hair and could cause yellowing. Whether used at the roots to support growth or along the lengths to nourish, apply only a few drops.

• Wash hair thoroughly after applying castor oil.

  According to some reports, an extended leave-on time or an inadequate shampooing afterwards may encourage hair yellowing. To prevent this, focus on areas treated with castor oil during washing and rinse under running water. To ensure complete removal of castor oil residues, some people use two consecutive shampoo treatments.

• Avoid pure castor oil on coloured hair.

  As a precautionary measure, individuals with coloured hair may consider replacing castor oil with another plant oil to care for their hair, such as hibiscus oil or mustard oil. Another option is to dilute castor oil in another plant oil to a 20 per cent concentration. This allows one to benefit from castor oil’s hair-supporting properties without risk.

• Verify the quality of the castor oil used.

  To limit the risk of hair yellowing, it is recommended to use castor oil of high quality, cold-pressed and filtered to prevent any trace of impurity from adhering to hair fibres.

• JOEKES I. & al. Yellowing and bleaching of grey hair caused by photo and thermal degradation. Journal of Photochemistry and Photobiology B: Biology (2014).

• MARWAT S. K. & al. Review - Ricinus communis - ethnomedicinal uses and pharmacological activities. Pakistan Journal of Pharmaceutical Sciences (2017).

• TESSEMA S. S. & al. Physicochemical characterization and evaluation of castor oil (R. communis) for hair biocosmetics. American Journal of Applied Chemistry (2019).