Both codeine and morphine work by attaching to the same receptors in the brain and spinal cord to block pain. Additionally, the body naturally converts a portion of codeine into morphine after you swallow it. Taking both means you are essentially taking a double dose of morphine, which can overwhelm the central nervous system.
This is a companion discussion topic for the original article at https://patient.info/treatment-medication/codeine-and-morphine-interaction