Cantharis is primarily used to treat cystitis, which is inflammation of the urinary bladder because of infection or irritation. It is also used to treat burns and blisters . Spanish fly was traditionally used as an aphrodisiac (increases sexual desire). It is also used for acute inflammations of the mucosae, the urinary orgasms, the reproductive organs, the gastrointestinal tract, and the skin with formations of blisters, effusions in cavities of the body, minor burns and scalds.