a world health organization ( who ) method has been developed which provides for drugs to be administered immediately if there is pain , to be given 'by the clock ' rather than 'on demand ' and to be increased from non-opioids ( aspirin or paracetamol ) to weak opioids ( codeine ) and then to strong opioids ( morphine ) until the patient is free from pain -- hence the concept of a three-step ladder for cancer pain relief .
C0030193
pain
C0009214
codeine