Which topical anesthetic has the slowest onset of roughly 20 minutes?

Which topical anesthetic has the slowest onset of roughly 20 minutes?

• A. Benzocaine
• B. Lidocaine
• C. Dyclonine HCl
• D. Tetracaine HCl

Answer: (D)

Onset of topical anesthesia depends on how quickly the drug can diffuse through the mucosa to reach the nerve membranes and block conduction. This diffusion is governed largely by how ionized the drug is at tissue pH and how lipid-soluble it is. A drug that remains more ionized at physiological pH crosses membranes more slowly, so its onset is delayed. Tetracaine has the slowest onset among these because it has chemical properties that make it more ionized at tissue pH and highly potent with strong protein binding. That combination slows its diffusion into nerve membranes, producing an onset around 20 minutes, even though it lasts longer. Benzocaine tends to diffuse into tissue more readily, giving a quicker onset. Lidocaine has a moderate onset, and dyclonine, while longer-acting than benzocaine, generally produces anesthesia faster than tetracaine due to its different pKa and diffusion characteristics.

Onset of topical anesthesia depends on how quickly the drug can diffuse through the mucosa to reach the nerve membranes and block conduction. This diffusion is governed largely by how ionized the drug is at tissue pH and how lipid-soluble it is. A drug that remains more ionized at physiological pH crosses membranes more slowly, so its onset is delayed.

Tetracaine has the slowest onset among these because it has chemical properties that make it more ionized at tissue pH and highly potent with strong protein binding. That combination slows its diffusion into nerve membranes, producing an onset around 20 minutes, even though it lasts longer.

Benzocaine tends to diffuse into tissue more readily, giving a quicker onset. Lidocaine has a moderate onset, and dyclonine, while longer-acting than benzocaine, generally produces anesthesia faster than tetracaine due to its different pKa and diffusion characteristics.