Fine and medium hair should smooth castor oil through their mid-lengths and ends, leaving it on for up to 30 minutes before shampooing. Coarse, thick, and ultra-dry hair types, on the other hand, can leave castor oil in as an overnight treatment or smooth it over their ends as a leave-in.
There are no exact guidelines for how long you should leave castor oil on your hair. That said, know that leaving it in too long can actually dry out your hair and scalp. To prevent drying, wash the castor oil from your hair after about two hours.
This type of treatment can improve hair's elasticity, boost shine and help to reduce frizz. Depending on your hair type and the product's directions, you can leave it on for anywhere from 20 minutes to overnight.
A common side effect of using castor oil on hair is acute hair felting. This is a serious condition, where the hair twists and entangles and turns into hard mass like a stone that resembles a bird's nest. This condition, if started once, is hard to get rid of.
According to Garodia, oil helps strengthen the hair shaft, especially in case of frizzy and dry hair. It's most beneficial when the oil is left in the hair overnight.
Whether you are trying to avoid chemicals or are just looking to experiment with some new hair care products, try natural oils such as coconut, argan, jojoba, almond, olive and grapeseed. These are some of the best oils for hair health and can be used directly on the skin and hair.
Avoid oiling your hair if you have an oily scalp
“There are high chances of getting an infection if you apply oil frequently on an oily scalp. You can do it once in a blue moon. Also, if you have acne on your forehead then avoid applying oil to the crown region,” she suggests.
Being very astringent in nature, castor oil can cause irritations to the skin. This could further lead to dryness, sensitivity, and even loss of hair and that's clearly not something you would want.
The tl;dr: no. "Castor oil will not grow hair," says Perry Romanowski, a cosmetic chemist and the author of Beginning Cosmetic Chemistry. "There is no evidence for it and no scientific theory supporting that it would work. So yes, it's a total myth."
How often can you use it: Use once per week for 20-30 minutes as a treatment. It is an oil, so a little goes a long way. Works well with: Carrier oils such as coconut (or other ingredients that impart moisture to hair). Don't use with: There are no known ingredients that negatively interfere with castor oil.
Prevents Hair Fall
Massaging your scalp with castor oil helps make the roots stronger and prevent hair fall. It has the ability to trigger and stimulate hair growth from dormant follicles.