Which scenario increases the risk of methemoglobinemia with prilocaine?

• A. High-dose prilocaine or repeated dosing in susceptible individuals.
• B. Low-dose prilocaine given once.
• C. Concurrent use of vasoconstrictors.
• D. Prilocaine never causes methemoglobinemia.

Answer: (A)

The risk of methemoglobinemia from prilocaine rises when more drug spills into systemic circulation and is metabolized to its oxidizing byproduct. Prilocaine’s metabolite, o-toluidine, can convert hemoglobin from the ferrous to the ferric state, forming methemoglobin and impairing oxygen delivery. This effect is amplified in individuals who are particularly susceptible to oxidative stress or who have limited capacity to reduce methemoglobin back to normal hemoglobin. Therefore, taking a high dose or receiving repeated dosing of prilocaine in these vulnerable individuals significantly increases the chance of methemoglobinemia. In contrast, a low dose given once to a healthy person carries a much lower risk, and while using a vasoconstrictor with the anesthetic changes absorption, it does not by itself markedly raise methemoglobinemia risk. Also, prilocaine can cause methemoglobinemia under certain conditions, so the statement that it never causes it isn’t accurate.