According to Haley and Rhodes, neostigmine bromide (alternatively known as Prostigmine) has an LD50 in mice of around 0.165 mg/kg by IV injection. Pubchem claims that this is also the LD50 for neostigmine iodide. According to Pubchem, Ro-1837, the analog of neostigmine in which the ester oxygen is replaced with sulfur, having the chemical name (3-Mercaptophenyl) trimethylammonium iodide dimethylcarbamate, has an IV LD50 of 18 mg/kg. Also according to page 209 of the book Chemical Warfare Agents and Related Chemical Problems, other S-thiocarbamates are likewise significantly less toxic than carbamates (ie. TL-1216 (subcutaneous LD50 of 0.17 mg/kg) vs TL-1239 (Subcutaneous LD50 somewhere over 80 mg/kg) ). Why is this so?
Haley, T. J., & Rhodes, B. M. (1950). A note on the acute toxicity of neostigmine methyl bromide in the rat. Journal of the American Pharmaceutical Association (Scientific Ed.), 39(12), 701. doi:10.1002/jps.3030391218