The syrupy poison, diethylene glycol, is an indispensable part of the modern world, an industrial solvent and prime ingredient in some antifreeze. It is also a killer. And the deaths, if not intentional, are often no accident. Over the years, the poison has been loaded into all varieties of medicine - cough syrup, fever medication, injectable drugs - a result of counterfeiters who profit by substituting the sweet-tasting solvent for a safe, more expensive syrup, usually glycerin, commonly used in drugs, food, toothpaste and other products. Toxic syrup has figured in at least eight mass poisonings around the world in the past two decades. Researchers estimate that thousands have died. In many cases, the precise origin of the poison has never been determined. But records and interviews show that in three of the last four cases it was made in China, a major source of counterfeit drugs. Panama is the most recent victim. Last year, government officials there unwittingly mixed diethylene glycol into 260,000 bottles of cold medicine - with devastating results. Families have reported 365 deaths from the poison, 100 of which have been confirmed so far. Panama's death toll leads directly to Chinese companies that made and exported the poison as 99.5 percent pure glycerin.