The oxide and carbonate of magnesium are also invaluable as antidotes, since they form insoluble compounds with oxalic acid and salts of mercury, arsenic, and copper.
In these cases lime-water, alkalis and magnesia should be used as antidotes, and opium may be required.
An electuary of opium, known as Mithradatum, was invented by Mithradates VI., king of Pontus, who lived in constant fear of being poisoned, and tested the effects of poisons on criminals, and is said to have taken poisons and their antidotes every day in the year.
The toxins produced by microbes, if too weak to destroy the leucocytes, induce them to secrete antitoxins, which not only act as antidotes to the toxins and are injurious to the microbes, but also increase the phagocytic power of the leucocytes (opsonius of Wright).
The antagonism between certain drugs has been much studied in relation to their use as antidotes in poisoning, the aim being to counteract the effects rather than to obtain a direct physiological antagonistic action.