The veterinary doctor Maurice Crowther Hall (1881-1938) discovered in 1921 that carbon tetrachloride was incredibly effective as an anthelminthic in eradicating hookworm via ingestion. In one of the clinical trials of carbon tetrachloride, it was tested on criminals to determine its safety for use in human beings. Beginning in 1922, capsules of pure carbon tetrachloride were marketed by Merck under the name Necatorina (variants include Neo-necatorina and Necatorine). Necatorina was used as a medication against parasitic diseases in humans. This medication was most prevalently used in Latin American countries. Its toxicity was not well understood at the time and toxic effects were attributed to impurities in the capsules rather than carbon tetrachloride itself. Due to carbon tetrachloride's toxicity, tetrachloroethylene (which was also investigated by Hall in 1925) replaced its use as an anthelmintic by the 1940s.