Large-scale chronic arsenic poisoning in several areas worldwide has not been eradicated. Adverse health effects of inorganic arsenic (iAs) exposure are associated with decreased arsenic methylation capacity (AMC). The AMC results from 21 children and 39 adults with acute arsenic poisoning support and effectively guide the analysis and evaluation of AMC in chronic arsenic poisoning. Results of the 65th-year follow-up for 6,223 cases of infants, who survived subacute arsenic poisoning, contribut to future projections against health effects (cancer and cognitive dysfunction). Currently, arsenolipids from fish and shellfish are attracting attention because of their direct association with brain dysfunction. Accordingly, the European Food Safety Authority is considering legal restrictions on their dietary intake. Modern society overlooks the environmental contamination and health hazards caused by arsenic chemical weapons mainly produced during World War II. This review aims to evaluation the various health effects of AMC reduction in acute, subacute, and chronic arsenic poisoning, in addition to the methylarsenic compounds such as arsenolipids, arsenosugars, and arsenobetaine, as well as arsenic chemical weapons like lewisite, diphenylchloroarsin, and diphenylcyanoarsine.