Morphine is used for dyspnea in terminally ill cancer patients by : Both endogenous (b-endorphin) and exogenous (morphine) opioids modulate the perception of dyspnea by binding to opioid receptors. Proposed mechanisms whereby opioids relieve refractory dyspnea include decreasing respiratory drive with an associated decrease in corollary discharge; altering central perception; altering the activity of peripheral opioid receptors located in the lung and decreasing anxiety. Ref KD Tripathi 8th ed