What are the effects of arsenic?
(Q&A by Cherie Tryon)

Arsenic is present in two forms, organic and inorganic. Some arsenic-containing organic compounds are used as drugs and pesticides. Arsenic was actually used in the past to treat diseases like syphilis. Poisoning from organic arsenic can occur from a daily ingestion of just a few micrograms. It affects several body systems including cardiovascular, nervous, and hematopoietic systems. It can also affect the skin. A fatal dose for adults is 70 mg to about 180 mg. So when you hear in dramatic plays that the actor or actress used arsenic to kill their enemy, this could actually be the case.
Inorganic compounds of arsenic normally effect humans when it is inhaled. A smelter worker or a sprayer of arsenic-containing pesticides could inhale a great amount of arsenic. The initial symptoms include diarrhea and vomiting. This can be followed by a state of shock from the dehydration.
If you were to get arsenic poisoning, there’s hope. You would be treated with 2.5 mg to 5 mg of dimercaporal per kilogram of body weight every four hours for the first two days. It would then be reduced to two injections on the third day and then daily for about the next five days.
Arsenic is not a joke. It is a serious poison that can be fatal. If there is a high concentration in groundwater, you could get cancer if you drank it for twenty to thirty years. Arsenic causes cancer of the bladder, lungs, kidneys and skin. Brief or occasional exposure will not harm you unless it is in very, very high concentrations.

source: http://www.montanaforum.com/rednews/2002/09/09/build/pollution
http://crystal.biol.csufresno.edu:8080/projects/122.html