Can You Really Wash Your Hair with Castile Soap? We Asked The Experts

Castile Soap

Type of ingredient: Natural, low-ingredient, vegetable oil-based cleansing soap.

Main benefits: Contains minimal ingredients with a low risk of sensitivity, effectively cleanses the scalp.

Who should use it: Those with sensitivity to chemicals or ingredients found in traditional shampoos. According to Gaunitz, it may also be beneficial for scalp issues such as dermatitis, psoriasis, and dandruff.

How often can you use it: Varies, based on your scalp condition and hair type.

Works well with: While castile soap is oil-based, Kayganich says that it can leave hair feeling more matted and tangled, so pairing it with a good conditioner is necessary.

Don't use with: Kayganich says to avoid using castile soap as a shampoo if you have hard water. It can cause a potential chemical reaction that greatly ups the likelihood of ending up with leftover residue and soap scum.

Benefits of Castile Soap for Hair

It's important first to understand how castile soap differs from traditional shampoos. "Typical shampoos are chemical-based with a lower pH and contain ingredients that leave some residue behind that often coats the hair to add shine or help with manageability," explains Gaunitz. Castile soap has a very high pH (more on that in a moment) and contains very minimal ingredients and no chemicals. To that point...

• Is an effective choice for those with easily irritated or sensitive skin: Because of the (very) few ingredients in castile soap, it's a good choice for those who have a low tolerance to chemicals or allergies or sensitivities to ingredients found in traditional shampoo formulas, says Gaunitz.
• Hydrates the scalp with its oil-base: All castile soaps have some vegetable oil base. "They originated in Castile, Spain, and were first made with olive oil. Today, it's also made with other oils such as hemp, coconut, or neem, just to name a few," says Kayganich. Coupled with the fact that it's free of often-drying and stripping sulfates (again, more on that point to come), it can help keep your scalp healthy, she says. (And this, in turn, is a good thing for your hair as a healthy scalp equals healthy hair, she adds.) Gaunitz expands on that point, noting that the oil-base, coupled with the high pH, makes it a good option for scalp issues such as dandruff or dermatitis, though it may not necessarily have much of an effect on those with normal scalps.

And, while not a hair benefit, per se, Kayganich adds that another nice thing about using castile soap is that it's biodegradable and doesn't pollute the water supply.

Hair Type Considerations

Both experts agree that anyone with colored hair should not use castile soap in place of shampoo. It goes back to the pH factor; a lower pH level helps preserve hair color, and, as mentioned, castile soap has an extremely high pH. "Despite the fact that it is a gentle cleanser, it's very clarifying and can end up stripping your hair color," warns Kayganich.

Besides that, our experts were split. Kayganich says that it's good for all hair types and textures. She notes that she even loves using it on her own natural curls, thanks to the fact that it's free of drying sulfates and heavy waxes or silicones that can weigh down the hair. Gaunitz disagrees, saying he'd only recommend it for someone who has lots of sensitivities to ingredients or issues with their scalp, citing that, for many, washing with castile soap can leave their hair unruly and unmanageable. To that point...

How to Use Castile Soap for Hair

If you want to consider using castile soap for your hair, there are a few essential things to keep in mind. It requires a more focused cleansing M.O. that's not quite as straightforward as the simple sudsing you may be used to with your usual shampoo. Castile soap is very concentrated, and a little goes a long way; Kayganich underscores the importance of using a small amount and diluting it first before applying it. "When you add water to the castile soap, it creates an electrical attraction, where the atoms can help lift and remove dirt and oil from the scalp," she explains. You can experiment with the exact ratio of castile soap-to-water to find the right dilution for you and your hair, but the universal rule of thumb is to use an empty bottle to dilute the product before using it.

When you apply it, Kayganich suggests focusing on the roots and scalp rather than the lengths and ends of your hair. Given that castile soap can admittedly lead to tangles and snarls, moving downward as you "shampoo" can help prevent the hair from becoming knotted (as can brushing your hair out before you wash it, FYI). In other words, don't pile it all up on top of your head and go to town vigorously rubbing and lathering. And finally, make sure to always follow with a hydrating conditioner, Kayganich says.

The bottom line? Castile soap is amazing for so many different things, but it's not necessarily a fail-safe when it comes to being a shampoo substitute. Avoid it if you have colored hair, and make sure you understand what you can do to combat the possible drawbacks if you are going to use it—i.e., diluting it, using the right cleansing technique, and always pairing it with conditioner.

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