Is conjugated with glucuronic acid in the liver. Morphine-6-glucuronide is a very potent analgesic, whereas the conjugate at position 3 (morphine-3-glucuronide) has been found not to have opioid activity, but is believed to cause the neuro-excitatory effects seen with high doses of morphine. The conjugates are excreted primarily in urine, with small quantities appearing in bile. The duration of action of morphine is 4 to 6 hours when administered systemically to morphine-naïve individuals but considerably longer when injected epidurally, because its low lipophilicity prevents redistribution from the epidural space. [Note: A patient’s age can influence the response to morphine. Elderly patients are more sensitive to the analgesic effects of the drug, possibly due to decreased metabolism or other factors, such as decreased lean body mass, renal function, etc. They should be treated with lower doses. Neonates should not receive morphine because of their low conjugating capacity.]