Toxicant any substance that causes an adverse biological effect, toxic response. Drug- pharmacological effect (use toxic effect for benefit ex. chemotherapy) Human made chemicals perceived to be more dangerous but have lots of testing, well researched, many trials compared to natural products perceived to be safe. Endogenous substances normal components in body required or not. Xenobiotics chemicals not normally present in the body (exogenous) Toxicology principle #1-all substances natural or man-made our poisons (toxicants), the dose makes the poison. Toxicology principle #4 dose response increase with increased dose (a graded response) as dose increases a) in individuals: effected increase, symptoms more severe, death; b) in a population number affected increases. Dose response group exceptions are immunological reactions things system can handle. Hormesis stimulates biological systems at low doses, inhibit them high doses, non-nutritional substances have beneficial effects at low doses, implications of low dose extrapolation ex. radiation, alcohol, fluorine.