According to the Castile Soap website, Castile soap originated in the Castilla region of Spain and quickly became popular throughout Europe. The main---and sometimes only---ingredient in Castile soap is olive oil, which makes this firm, white soap a natural, mild cleanser perfect for people with sensitive skin or dermatological conditions. While Castile soap alone cannot cure acne, it could be a beneficial addition to your skin care regimen.
A 1999 article on the history of soap published in "The Pharmaceutical Journal" describes only two ingredients found in traditional Castile soap: plant ash and pure olive oil. Today, many companies sell Castile soaps based on a variety of recipes, but quality products will contain little---and sometimes nothing---apart from olive oil. This level of purity not only enhances the quality of the soap, but it also reduces the likelihood of skin irritation or harmful reactions with other skin care products or topical medicines.
Pure Castile soap is one of the mildest cleansers available. In fact, the Castile Soap website recommends it for people with sensitive skin, allergies to other common soap ingredients, psoriasis, rosacea or eczema. It is also safe and gentle enough to use when bathing infants. This means that while Castile soap may not significantly improve your acne, it certainly shouldn't make the problem worse or create any additional issues.
An article on the Acne Treatment News website recounts the experiences of several individuals who experimented with olive oil soap as a means of acne prevention. The article reports that poor-quality soap appeared to worsen the users' acne, but several higher-quality soaps, which were presumably more pure, led to noticeable improvements. The article also describes possible uses of pure olive oil to clean and moisturize the skin.
Simply washing your face with Castile soap may help with acne, but combining this habit with other forms of skin care may be more effective. The AZWellness website recommends washing with Castile soap two to three times daily as part of a more comprehensive treatment regimen, including consuming diluted lemon juice, applying natural oils to the skin and making specific dietary changes. The Middle Path Natural Body Care site suggests diluting liquid Castile soap with an equal amount of water, washing with the mixture and applying an astringent such as witch hazel afterward.
While Castile soap is generally mild and should be safe for most uses, you should consult with a dermatologist or another trusted medical professional before making dramatic changes to your personal care habits or acne treatments. If use Castile soap to prevent or treat acne, evaluate its effectiveness after several days of use. You may see immediate results, but a cleansing routines often require some time to have a noticeable effect.