Which property best explains fentanyl's rapid onset after administration?

• A. Hydrophilicity
• B. Large molecular weight
• C. Extremely lipophilic, with high logP and large distribution volume
• D. Low receptor affinity

Answer: (C)

Rapid onset hinges on how fast the drug reaches the brain, where opioids exert their effect. Fentanyl is highly lipophilic, so it readily crosses lipid membranes and the blood–brain barrier, allowing brain concentrations to rise quickly and producing fast analgesia. Its high logP and large distribution volume reflect this easy tissue partitioning, especially into the brain. In contrast, hydrophilicity would slow entry into the brain, a large molecular weight can hinder diffusion, and low receptor affinity would mainly affect potency rather than how fast the drug acts. So the extreme lipophilicity best explains why fentanyl works so quickly.