Certain drugs antidote each other therapeutically, because they produce similar effects locally in certain parts of the organism or on certain tissues and functions or generally throughout their action as a whole. The antidotal relation is based therefore on similarity and is operative according to the law of cure, similia similibus; and again the antidotal relationship between drugs may be general or partial, according as the similarity in their action is general or confined to certain parts only. Thus camphor antidotes the effects of cantharis only so far as these concern the mucous membrane of the urinary tract, while the same tissue elsewhere is not antidoted by it.

Such antidotal relation is of use in practice, by which we can modify or annul an undesirable action of a drug, for instance, Anacardium bears an antidotal relation to Rhus, especially in its action on the skin, Hepar to Mercury, Chamomilla to Coffea and Pulsatilla, etc. An interesting phase of the antidotal relationship is the mutual antidotal or at least modifying power of the higher and the lower attenuations of the same drug, as well as the antidotal relationship between the chronic effects of the crude drug and the attenuated drug, as is seen in treating chronic tobacco poisoning with Tabacum high. This holds true at times in acute conditions as has been frequently verified in poisoning with Rhus where a high attenuation will prove the quickest antidote.