Presentation and Properties

Morphine is a naturally occurring opiate, from the opium poppy (Fig 1). It comes in 1 ml ampoules containing a clear solution of 10 mg/ml. For intravenous use, this is normally diluted with saline to give a solution of 1 mg/ml. Morphine is less lipid-soluble than fentanyl.

Pharmacodynamics

Morphine is less potent than fentanyl (Fig 2). Analgesia is the clinically relevant effect.

Pharmacokinetics

Table 1 shows the physicochemical properties of morphine and fentanyl. Morphine has a slower onset of effect than fentanyl. It is less lipid-soluble, so crosses the blood-brain barrier less rapidly than fentanyl. Morphine has a longer-lasting effect than fentanyl when used as a bolus dose. Its plasma concentration falls relatively rapidly due to distribution to vessel-rich tissues. Metabolism occurs in the liver; an active metabolite is morphine-6-glucuronide, which is more potent than morphine. The presence of an active metabolite is only important in patients with renal failure who receive repeated doses of morphine.