Except as provided in subsection (2) of this section, any pharmacist filling a prescription under a state purchased health care program as defined in *RCW 41.05.011(2) shall substitute, where identified, a preferred drug for any nonpreferred drug in a given therapeutic class, unless the endorsing practitioner has indicated on the prescription that the nonpreferred drug must be dispensed as written, or the prescription is for a refill of an antipsychotic, antidepressant, antiepileptic, chemotherapy, antiretroviral, or immunosuppressive drug, or for the refill of a immunomodulator/antiviral treatment for hepatitis C for which an established, fixed duration of therapy is prescribed for at least twenty-four weeks but no more than forty-eight weeks, in which case the pharmacist shall dispense the prescribed nonpreferred drug.