The chemical structure of fentanyl allows rapid brain entry because it is ...?

• A. Very hydrophilic, low lipophilicity
• B. Extremely lipophilic, with high logP and large distribution volume
• C. Very large and rigid, hindering crossing membranes
• D. Highly polar, with poor membrane permeability

Answer: (B)

Rapid brain entry hinges on the ability to cross the blood–brain barrier by passing through lipid membranes. Molecules that are highly lipophilic readily partition into these membranes and diffuse into the CNS. Fentanyl’s chemical structure makes it extremely lipophilic (high logP), so it quickly crosses into the brain after administration. Its large distribution volume reflects extensive tissue distribution, including the brain, which further supports a rapid onset of central effects. If a compound were very hydrophilic, highly polar, or too large/rigid, it would struggle to cross membranes and would enter the brain much more slowly. Thus, extreme lipophilicity with high logP and a large distribution volume best explains fentanyl’s fast brain entry.