What Is Castile Soap, the Superstar Cleanser for Both Home and Body

You may have heard of castile soap before (hello, Dr. Bronner's), but do you know what makes it different from other traditional soaps on the market? There are many factors that make castile soap wildly different from your traditional body wash and detergent, so it's no wonder the multipurpose cleaner has earned its rank amongst the most popular household items today. If you're new to the soap or simply unsure how to use it, we asked experts about everything you need to know about this ancient formula.

What Is Castile Soap?

Castile soap originated with olive oil from the Castile region of Spain hundreds of years ago (hence the name). Since then, it has expanded to include a variety of vegetable oils. "Although castile soaps were historically produced from olive oil, most castile soaps today are made primarily with coconut oil, which is more abundant and less costly than olive oil," says cosmetic chemist Kelly Dobos. Although castile soap is most often available in liquid form, it can also be found in bars.

The best way to think about castile soap is as "true soap", meaning it's the product of reacting natural oils and fats with a base like sodium or potassium hydroxide. "Many consumers use the term soap to refer to different cleansing bars and liquids today, including those made primarily from synthetic surfactants. Most of what we refer to as 'soap' today is detergent—detergents are not biodegradable," says Joshua Onysko, founder of Alpine Provisions. On the other hand, castile soaps are naturally derived and therefore easily biodegradable.

Castile Soap Uses

The amazing thing about castile soap is that it can be used for many things. "Castile soap can be effectively used for cleansing the body and hair—but can also be used as an all-purpose cleanser for household cleaning, washing clothing, and cleansing makeup brushes and sponges," says Dobos. "That being said, castile soap needs to be diluted prior to use since it is very strong."

Although using castile soap to clean the body (we love Alpine Provisions Castile Body Wash; $15, amazon.com) or home (check out Dr. Bronner's Pure-Castile Liquid Soap; $38 for 2-pack, amazon.com) are the most common ways this product is used, there are other ways people like to use this ancient soap formula. For example: "Diluted castile soap solutions can even be used in the garden to help get rid of small, soft-bodied mites or aphids," says Dobos. "Just note that some plants, like tomatoes, can be sensitive to soapy sprays, so you have to do a bit of research first."

You can also use a small dish of apple cider vinegar-and-soap solution to help rid your kitchen of fruit flies. "The vinegar attracts the flies and the soap reduces the surface tension of the water so that the bugs slip in and can't escape," says Dobos.

Benefits of Castile Soap

For starters, castile soaps are gentle yet highly effective cleansers on skin. Even though it can tackle the most stubborn grime, it's also very hydrating due to saponified oils, meaning your skin won't feel overly stripped and squeaky. "However, it's always a good idea to review the ingredient lists for any possible allergens if you're using it on the skin, and always discuss any specific questions with a dermatologist or physician before use," says Onysko. Also, keep in mind that castile soap itself is not antibacterial and does not kill microbes, although it will wash away dirt and germs.

Another plus for vegans: One of castile soap's distinguishing traits is that it's made purely of vegetable fat, rather than animal fat. Plus, you are using a highly concentrated formula of soap, so you can use less and reduce waste. "The natural saponification process that castile soap follows has been used for thousands of years to make soap that helps to keep us safe from germs and viruses," explains Onysko. "And the process, as well as the end result, is kinder to our bodies and the environment."

Side Effects of Castile Soap

Because it is a true soap, castile tends to have a higher pH, which can be harsher and drying with prolonged skin contact—like if you were maybe using it to wash a sink full of dishes. "It also does not perform as well in hard water conditions and can form deposits in tubs and showers, which will gradually take on a gray appearance as the dirt and scum become embedded," explains Dobos.