Yes, cornstarch is generally considered safe for use on the underarms. It is a natural ingredient derived from corn and is commonly used as a thickening agent in cooking. When applied to the underarms, cornstarch can help absorb excess moisture and reduce friction, which can be beneficial in preventing chafing and irritation. Additionally, cornstarch is generally non-irritating and non-comedogenic, meaning it is unlikely to cause skin irritation or clog pores. However, as with any skincare product, individual reactions may vary, so it's always a good idea to patch test a small area of skin before applying it more widely.