Re: How much arsenic (in grams or milligrams) can be poisonous?

Arsenic (As) is a metalloid element in the Nitrogen family (5A). According to the sources listed below, Arsenic is both toxic and carcinogenic. This means that exposure below the lethal dosage is considered a cancer hazard if it is above a specified exposure limit. There are two numbers that can be used to define the lethal dose of a substance: the LD₅₀ defines the oral lethal dose that will kill 50% of the tested animals that eat the substance; and the LC₅₀ defines the inhaled lethal concentration that will kill 50% of the tested animals breathing the substance. For arsenic, the LD₅₀ is between 15 and 30 mg/kg (milligrams of arsenic per kilogram of body weight), and the LC ₅₀ is around 400 mg/m³/2H (milligrams of arsenic per cubic meter of air after two hours of exposure). The exposure limit for avoiding cancer risks associated with arsenic is 2 ppm ("parts per million", which is equivalent to mg/m³) - exposure to arsenic levels between 2 ppm and 400 ppm can be carcinogenic and possibly lethal depending on how long the person, or animal, is exposed.

MSDS for Arsenic from JT Baker
Intl. Chem. Safety Card for Arsenic
MSDS for Arsenic from Fisher Scientific

As an aside, you mentioned conflicting values "from 65 milligrams to 1/10 of a gram"; 1/10 grams is the same as 100 mg, which is not that different from 65 mg as a dosage. For a bodyweight around 70 kg (155 lb.), the lethal dose of arsenic would be closer to a gram than 100 mg.