The levels of various metabolites of the methylation pathway are important for protection from side effects of anesthesia. As early as 1942 it was recognized that the addition of methionine is preventative for side effects from the use of chloroform. Methionine affords protection from liver injury as a result of chloroform anesthesia. Methionine also protects against effects of nitrous oxide anesthesia. Nitrous oxide disrupts the activity of methionine synthase, a central enzyme in the methylation cycle. Again, preloading with methionine appears to accelerate recover and reduce side effects associated with this form of anesthesia.