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.
It is a law of drug action, according to which the administration of each medicine causes, at first, certain abnormal symptoms, the so-called primary effects of medicines, but afterwards, by reaction of the organism, a condition entirely the opposite, where this is possible, of this first effect is produced - the secondary effects, for instance, narcotic substances produce primarily insensibility and secondarily pain. In order to produce the primary effects, material doses are required.
In his essay, entitled "Suggestions for Ascertaining the Curative Powers of Drugs," Hahnemann says: "Most medicines have more than one action; the first a direct action, which gradually changes into the second (which I call the indirectly secondary action). The latter is generally a state exactly the opposite of the former. In this way most vegetable drugs act. But few medicines are exceptions to this rule, i. e., metals and minerals. The thorough examination of drug provings, as in our possession at present, does not justify any division of drug-symptoms into primary and secondary. There are indeed in every proving, as Dr. Hering has shown, primary and secondary symptoms, in the sense that some symptoms appear earlier and others later in the course of the proving, but although these may appear opposed to each other, they are all to be regarded as drug symptoms, and, as such, indicate the remedy.