Which statement correctly describes the methemoglobinemia risk when using cyanide antidotes?

• A. No effect on methemoglobin
• B. Sodium nitrite increases methemoglobin levels
• C. It decreases methemoglobin
• D. It increases blood oxygen affinity

Answer: (B)

Cyanide antidotes that work by scavenging cyanide use a mechanism that relies on forming methemoglobin, which can bind cyanide and help detoxify it. Sodium nitrite does exactly that: it oxidizes hemoglobin from Fe2+ to Fe3+, creating methemoglobin and raising methemoglobin levels. This is why there is a methemoglobinemia risk when using nitrite-based antidotes—the higher the methemoglobin, the more cyanide can be sequestered, but at the cost of reduced oxygen-carrying capacity. The other statements miss this key point: nitrite-containing antidotes do affect methemoglobin by increasing it, they do not decrease it, and they do not increase oxygen affinity in a beneficial way—methemoglobinemia actually impairs oxygen delivery to tissues.