morphine: a white crystalline alkaloid derived from the opium poppy, Papaver somniferum, the source of its principal pharmacological activity. Morphine acts on the central nervous system to produce both depression and euphoric stimulation, effects that are mediated through its stimulation of mu opioid receptors. Stimulation of mu receptors exerts effects that depress nerve impulse transmission. Even in small amounts morphine depresses the respiratory system. Mu receptors' activation in some instances inhibits the transmission of inhibitory impulses, thereby causing stimulation. Examples of this include the euphoric response and constriction of the pupils by the third cranial nerve. Morphine has a marked analgesic effect because of its stimulation of mu receptors in both the brain and spinal cord, and its principal therapeutic value is for the relief of moderate to severe pain. Morphine rarely provides total relief of pain, but in most cases it reduces the level of suffering. Patients with severe pain may become drowsy and relaxed but seldom achieve the sensation of euphoria associated with use of the drug. Morphine is often given parenterally because its rapid absorption following oral administration is followed by rapid first-pass metabolism to an inactive glucuronide (oral bioavailability ranges from 17% to 33%). Repeated use of morphine leads to tolerance, necessitating increased dosage levels to get the same degree of pain relief.