While no studies support the idea that cornstarch can be used to treat rashes under the breasts, it is claimed that it is especially effective in reducing the burning and itching associated with a heat or sweat rash.
Start by cleansing the affected area with a gentle soap and water if you wish to try out this treatment for your breast rash. Then, wipe away any remaining moisture with a soft towel. Finally, apply cornstarch to the affected area when the skin has completely dried.