Thousands of patients in California and across the nation who take expensive prescription drugs every month for cancer, rheumatoid arthritis and other ailments are facing sticker shock at the pharmacy. Until recently, most of these patients typically paid modest co-pays for the advanced drugs. But increasingly, Anthem Blue Cross, Aetna and other insurers are shifting more prescriptions to a new category requiring patients to shoulder a larger share of the drug's cost. The result: Pharmacy bills are going up by hundreds of dollars a month—on top of insurance premiums.