Why do anesthetics easily enter into CNS and have quick effect?

• A. They are highly polar
• B. They are highly lipophilic
• C. They are rapidly metabolized
• D. They are bound to plasma proteins

Answer: (B)

Crossing into the brain mainly depends on lipid solubility because the blood–brain barrier is a lipid-rich barrier that favors diffusion of nonpolar, small molecules. Anesthetics are highly lipophilic, so they readily dissolve in and diffuse through brain cell membranes and into CNS tissue as blood flows through the brain. This rapid diffusion leads to quick brain concentrations and fast onset of action.

If a drug were highly polar, it would have trouble crossing the barrier, delaying CNS effects. Binding to plasma proteins would lower the freely circulating fraction available to reach the brain, slowing onset rather than speeding it. Metabolism affects how long the drug stays active, not how quickly it acts initially. So the quick CNS entry and effect come from their high lipophilicity.