A synergistic reaction occurs when two or more compounds interact for an effect on an organism greater than either compound has on its own, according to Sacramento State Office of Water Programs. Drug synergy occurs when combined drugs magnify the effects of both drugs, notes Wikipedia.
Synergism in toxicology refers to exposure to two or more chemicals causing health effects greater than either chemical would have caused alone, according to the Canadian Centre for Occupational Health and Safety. The organization notes that when chemicals are synergistic, the potential hazards should be evaluated. A phenomenon related to synergism is potentiation, when a substance that normally is not toxic is added to a chemical and makes the second chemical much more toxic.