This article discusses the use of biological markers for the diagnosis and monitoring of nitrous oxide (N2O) abuse. The authors propose the use of vitamin B12 as a biomarker for N2O intoxication. They explain that N2O exposure leads to the functional inactivation of vitamin B12, resulting in a decrease in methionine synthase activity. The authors suggest that plasma homocysteine and plasma MMA can be used as markers for recent N2O consumption and the clinical severity of N2O abuse, respectively. They recommend a combination of biomarkers and emphasize the importance of excluding other causes of elevated homocysteine or MMA levels. [Extracted from the article]