From these basic definitions, there are literally thousands of substances perpetrators can choose from to commit homicidal poisoning. It is much easier to detect these substances today thanks to modern scientific methods and advances. However, unless the death is unexpected or the circumstances questionable, it is unlikely extensive testing for more exotic substances will be performed. The following is a list of some of the common toxins that have been employed in homicidal poisonings over the centuries.

     Arsenic. Known as the King of Poisons and the Poison of Kings, this poison's reputation comes from its consistent use within the upper classes to poison family members. Arsenic is found in two basic forms, with both being extremely damaging to humans and other animals.

• Inorganic. This type comes in many forms. High concentrations may occur in groundwater caused by natural leaching or disruption of the earth's surface by mining activity. The two types which are most commonly found in groundwater are Trivalent and Pentavelant Arsenite. It is also found in minerals and ores. Arsenic compounds also were, until recently, used in pressure-treated wood as an anti-fungal and insecticide. Inorganic arsenic is also found in plastics.
• Organic arsenic is found in seafood and is easily metabolized in the body.

     Until the late 1700s, a reliable test for arsenic's presence did not exist. By the mid 1800s it was fairly easy to spot small amounts of arsenic in human tissue. However, problems remain with the discovery of arsenic and the diagnosis of murder because the presence of arsenic in the victim's remains may not necessarily indicate foul play. Conversely, when someone dies of deliberate arsenic poisoning, it may not be distinguishable from naturally occurring exposure.

     Arsenic has been used in many ways through the centuries. Arsenic Trioxide is currently used to treat acute leukemia and psoriasis. In the 18th, 19th and 20th centuries, arsenic was used to treat people with syphilis. The Union Army during the U.S. Civil War employed embalmers to preserve dead soldiers' bodies so they could be sent home. The practice proliferated and was soon popular across the Americas. Bodies were treated with numerous arsenic-based remedies ranging from a couple of ounces to more than 10 pounds of arsenic-based fluids. The use of arsenic-based embalming fluid stopped in 1910, but old cemeteries contain extremely high levels as does the groundwater around them, note John Konefes and Michael McGee on www.waterindustry.org. Modern victims of homicide buried in these areas may show high levels of arsenic, though their deaths are not attributable to poison. People drinking from sources of groundwater in the area also may be affected. Arsenic is a known cancer-causing agent, and in some areas, such as Bangladesh, the World Health Organization estimates cancer rates to be more than 200,000 in areas with arsenic-contaminated well water. Clearly the presence of arsenic does not necessarily point to a homicidal poisoning.

     Cyanide. This substance seems to be a favorite in mass homicides, suicides and politically motivated killings. Cyanide gas was used by both the Nazis and the United States in gas chambers. In the Jonestown murders of more than 900 people, cyanide was placed in a sweet drink and given to the members of the Peoples Temple led by Jim Jones. Cyanide is also a favorite of spies and other politically motivated groups. Undercover agents use it to kill themselves when capture is imminent. However, it is not a favorite for the average poisoner, most likely because it kills rapidly rather than mimicking an illness.

     Cyanide comes in a number of forms. The three most widely used and the most lethal are: hydrogen cyanide, sodium cyanide and potassium cyanide.

     Cyanide is found in insecticides and poisons intended for rodents. It is also a by-product of the combustion of fossil fuels and is found in cigarette smoke.

     A person who has been poisoned with cyanide may have the smell of bitter almond on their breath. But while approximately 60 percent of the population can detect this smell, the other 40 percent cannot. However, cyanide can be found at high concentrations in the victim's blood and urine if the samples are taken soon after exposure.

     Thallium. Another toxic heavy metal which is used by poisoners, this metal is soft and pliable and has a bluish tinge to it. It was used in ant and rat poison and as a treatment for ringworm.