Does castor oil cause hair yellowing?

The castor oil (INCI: Ricinus Communis (Castor) Seed Oil) is a plant oil obtained by cold pressing the seeds of the castor plant (Ricinus communis), a shrub cultivated in China, India, and Brazil. From an organoleptic perspective, castor oil appears as a thick yellow liquid and is noted for its nourishing and strengthening properties, which derive from its saturated fatty acids. These fatty acids share a chemical structure with cuticle lipids, allowing insertion by biomimicry to reinforce this layer’s cohesion. Moreover, although additional studies are needed, the castor oil could stimulate hair growth, a property that contributes to its popularity. This effect is due to its ability to inhibit prostaglandin D2 (PGD2), a protein involved in androgenetic alopecia.

Despite its many benefits for hair, castor oil faces criticism for its thick, viscous texture and its alleged yellowing effect. Nevertheless, this effect has never been reported or demonstrated in scientific studies.

Only a few reports mention the yellowing effect of castor oil when applied in large quantities and left on hair for long periods. It is hypothesized that this depends on natural hair pigmentation, light or bleached hair being more likely to pick up tint, their porosity, as porous hair absorbs more product on its surface, and on hair fiber oxidation, oxidized hair being more fragile. The apparent yellowing noted by some could also be an optical effect. Indeed, the castor oil has a natural yellow hue when applied to the hair, it may create a temporary yellowing effect, especially if the rinse is incomplete. However, without scientific studies on the subject, these are only hypotheses.