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.
Complementary Relation exists between drugs that complete a cure that is begun by another and carried to a certain point, where it is taken up by another drug and completed. If a remedy is allowed its full time of action, it will often lead up to a complementary remedy - that is, the symptoms remaining untouched, or brought to the surface, will often suggest a drug known to be complementary to the one given. This useful relationship of certain group of drugs is of great service in the treatment especially of chronic diseases. Such relationship exists between Belladonna and Calcarea; Apis and Natrum muriaticum; Aconi-tum and Sulphur; Chamomilla and Magnesia phos-phorica; Thuja and Silica.