The Lethal Dose 50 (LD50) test involves the administration of a substance to a group of animals at increasing doses in order to determine the dose that kills 50 percent of the test subjects within a set time frame. As defined here: http://safe2use.net/ca-ipm/00-07-05a.htm

“In adults, single doses above 10 grams or 140 mg/kg have a reasonable likelihood of causing toxicity. In adults, single doses of more than 25 grams have a high risk of lethality.”
http://www.druglib.com/activeingredient/acetaminophen

The LD50 of acetaminophen is pretty high -- I came across a bunch of conflicting estimates, but it looks like somewhere in the neighborhood of 800 milligrams per kilogram (2.2 pounds) of body weight ... but far smaller doses, especially if repeated, can cause severe liver damage as noted below:
http://www.medpagetoday.com/Gastroenterology/LiverTransplantation/14959

http://www.chem.purdue.edu/gchelp/116exams/e1f003.gif

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