Which property best explains fentanyl's rapid onset and high potency compared with morphine?

Which property best explains fentanyl's rapid onset and high potency compared with morphine?

• A. Higher molecular weight
• B. Greater lipid solubility
• C. Lower protein binding
• D. Slower metabolism

Answer: (B)

The main factor is how quickly the drug can reach the brain, which depends on lipid solubility. Fentanyl is highly lipophilic, so it crosses the blood–brain barrier rapidly and achieves high Central Nervous System concentrations quickly after administration, producing a rapid onset of analgesia and its high potency relative to morphine. Morphine, being less lipid-soluble, enters the brain more slowly, resulting in a slower onset and comparatively lower potency at the same dose. Higher molecular weight would impede membrane diffusion, not speed it up, so that doesn’t explain the rapid action. Lower protein binding would actually favor faster distribution but fentanyl is highly protein bound, so this isn’t the reason for its rapid CNS effects. Slower metabolism would affect duration more than onset; fentanyl’s fast CNS entry is the key reason for its rapid onset and high potency.

The main factor is how quickly the drug can reach the brain, which depends on lipid solubility. Fentanyl is highly lipophilic, so it crosses the blood–brain barrier rapidly and achieves high Central Nervous System concentrations quickly after administration, producing a rapid onset of analgesia and its high potency relative to morphine. Morphine, being less lipid-soluble, enters the brain more slowly, resulting in a slower onset and comparatively lower potency at the same dose.

Higher molecular weight would impede membrane diffusion, not speed it up, so that doesn’t explain the rapid action. Lower protein binding would actually favor faster distribution but fentanyl is highly protein bound, so this isn’t the reason for its rapid CNS effects. Slower metabolism would affect duration more than onset; fentanyl’s fast CNS entry is the key reason for its rapid onset and high potency.