Black castor oil has not been shown to grow hair in any controlled human trial. What it does reliably is coat the hair shaft, slow breakage, and keep the length you already grow, which looks like growth after three months of steady use.
The ancestral use runs centuries deep and the clinical file behind it is nearly empty.
Where the growth claim comes from
Castor oil is roughly 90 percent ricinoleic acid, a fatty acid with anti-inflammatory activity in lab and animal models. Jamaican black castor oil adds a roasting step that leaves ash in the oil and pushes the pH alkaline. What roasting changes in black castor oil is texture and color far more than chemistry.
The growth story rests on one thread. A computer modelling study suggested ricinoleic acid might block prostaglandin D2, a molecule found at raised levels in balding scalps. Nobody has shown that happening on a living scalp or counted new hairs after castor oil in a proper trial.
Hair grows about half an inch a month whether you oil it or not. Castor oil cannot speed that clock, but it can stop you losing length at the other end.
What the tradition actually did
In Jamaica, Haiti and across West Africa, black castor oil was a retention tool long before it was sold as a growth serum. Women worked it into hairlines and braids to protect strands under tension and hold moisture through dry seasons.
Retention deserves its real name. Calling it growth sets you up to feel cheated at month four when your hairline looks exactly the same.
How often to apply it
Once or twice a week is the working ceiling for the scalp. Black castor oil is among the heaviest oils you can buy, and daily use builds a film that traps shed skin and product. The full schedule for how often to oil your scalp depends on your hair type and wash routine.
Apply it as a pre-wash treatment. Warm a small amount between your palms and work it into the scalp with your fingertips for five minutes. Leave it for 30 minutes to overnight, then shampoo twice. A one-to-one mix with grapeseed or jojoba spreads further and rinses cleaner.
The fingertip work matters more than the oil. Scalp massage for hair growth has a small amount of human evidence behind it, which is more than castor oil can claim on its own.
Two cautions before you start
A 2017 case report in the International Journal of Trichology linked castor oil to acute hair felting, where long hair suddenly mats into a single mass that has to be cut out. It is rare, but a reason to apply less than you think you need.
Patch-test the inner arm first. Ricinoleic acid is a known contact allergen for a small number of people, and the roasted ash can irritate a sensitive scalp.
Who should bother
Black castor oil earns its place if you have coily or high-porosity hair that breaks before it reaches your shoulders. Skip it for fine, low-porosity or oily hair, where the film just sits on the surface.
Buy it as a sealant with a long history and use it that way. If a label promises regrowth, the label is doing marketing.