This section is from the book "A Compend Of The Principles Of Homoeopathy", by William Boericke. Also available from Amazon: A Compend of the Principles of Homoeopathy as Taught by Hahnemann.
Inimical Relation is the very opposite of the concordant and complementary. There seems to be a lack of harmony between certain drugs, as is also seen in certain chemical affinities. This may be so marked that when following each other in the treatment of a case, disturbance shows itself and the cure is interfered with and the whole case mixed up. Such a relation seems to exist between Apis and Rhus, between Causticum and Phosphorus; Mercurius and Silica; Sepia and Lachesis and others. Do not give these remedies after each other. It is well to note these inexplicable conditions of drug action, based on friendly or inimical relationship. If it be remembered that drugs are embodied forces, distinct entities with distinct powers to modify human life as manifested in functional activity and organic changes we can readily see that certain forces can work advantageously side by side; their joint result, thus following each other, being greater than either one singly; and again certain others cannot do so, but mere contact or propinquity upsetting the orderly progress of the case.
For further study see Boeninghausen, "The Sides of the Body and Drug Affinities".
Mohr, "The Inimical Relationship of Drugs".
Hering's contributions in the Archiv.
"Farrington's Clinical Materia Medica". Lecture I.