The more rapid onset and shorter duration of fentanyl relative to morphine is primarily due to which property?

The more rapid onset and shorter duration of fentanyl relative to morphine is primarily due to which property?

• A. Volume of distribution
• B. Hepatic clearance
• C. Renal clearance
• D. Lipid solubility

Answer: (D)

Lipid solubility governs how fast a drug reaches and leaves the brain. Fentanyl is highly lipophilic, so it rapidly crosses the blood-brain barrier and produces quick analgesia. Once it acts, its strong lipophilicity also drives rapid redistribution into tissues and fat, pulling the drug away from the CNS and shortening the duration of its effect after a bolus. Morphine, being less lipid-soluble, takes longer to enter the CNS and stays there longer, resulting in slower onset and a longer-lasting effect. While distribution and clearance influence overall exposure, the rapid CNS onset and brief duration of fentanyl compared with morphine are best explained by its lipid solubility.

Lipid solubility governs how fast a drug reaches and leaves the brain. Fentanyl is highly lipophilic, so it rapidly crosses the blood-brain barrier and produces quick analgesia. Once it acts, its strong lipophilicity also drives rapid redistribution into tissues and fat, pulling the drug away from the CNS and shortening the duration of its effect after a bolus. Morphine, being less lipid-soluble, takes longer to enter the CNS and stays there longer, resulting in slower onset and a longer-lasting effect. While distribution and clearance influence overall exposure, the rapid CNS onset and brief duration of fentanyl compared with morphine are best explained by its lipid solubility.