Which factor is the primary determinant of local anesthetic potency?

Which factor is the primary determinant of local anesthetic potency?

• A. pKa
• B. Molecular weight
• C. Lipid solubility
• D. Concentration

Answer: (C)

Penetration into the nerve membrane is what sets potency for a local anesthetic. The more lipid-soluble the drug, the better it partitions into the lipid bilayer surrounding the nerve cell and reaches the intracellular site of action—the voltage-gated sodium channel inside the axon. This means it can block conduction at lower concentrations, giving higher potency. While pKa influences how quickly the drug begins to work (the fraction that is non-ionized at physiological pH crosses membranes), and concentration determines the overall amount available, potency itself is best predicted by lipid solubility. Molecular weight has a looser, less consistent relationship, and concentration is about dose, not the intrinsic strength of the drug.

Penetration into the nerve membrane is what sets potency for a local anesthetic. The more lipid-soluble the drug, the better it partitions into the lipid bilayer surrounding the nerve cell and reaches the intracellular site of action—the voltage-gated sodium channel inside the axon. This means it can block conduction at lower concentrations, giving higher potency.

While pKa influences how quickly the drug begins to work (the fraction that is non-ionized at physiological pH crosses membranes), and concentration determines the overall amount available, potency itself is best predicted by lipid solubility. Molecular weight has a looser, less consistent relationship, and concentration is about dose, not the intrinsic strength of the drug.