Medical experts in Hawaii are warning patients that the state's general excise tax can be misapplied, potentially leading to overcharges and frustration for providers. Most medical providers set prices for their services, but insurance companies typically pay less than the listed price. The excise tax is supposed to be calculated based on what the provider is actually paid by both the insurance company and the patient, not on the original price the provider intended to charge. Dr. Kelley Withy, a medical school professor who advises doctors on charging Hawaii's 4.5 percent excise tax, said the tax adds a layer of confusion as it is a charge other states do not have. Most large providers, such as hospitals, are not required to pay the general tax.