The law of opposites is most appropriate in the area of regimen. If you eat too much, eat less; if you sleep too little, sleep more, etc. It is a suitable technique to find a place of moderation. Applied medicinally, the law of opposites seeks to remove the present symptoms by producing an opposing condition (ex. purgatives for constipation). This approach can give temporary relief in acute, self-limiting diseases but it cannot cure.
The law of similars is the law according to which medicine should be applied to cure disease. This approach does not cloak symptoms but removes the disease radically. The body cannot hold two similar diseases at the same time and the artificial disease (remedy) is always stronger, so it annihilates the existing disease.
Although the law of similars has been recognized by physicians from antiquity, with Hippocrates as its first recorded representative, it has often abandoned due to the aggravation this approach can trigger. The nature of medicine lies in its capacity to negatively affect the living organism at a dynamic level. All medicines must be poisons (although some may have low toxicity in their crude state or we may have become accustomed to their toxicity through long usage). Accordingly, they have to be administered in a controlled way or else it can wreak havoc: when you fight fire with fire, you can blow a gasket. It is out of fear of the “homeopathic aggravation” involved in using the law of similars that caused people to abandon this approach, even though they observed that this is how nature cures disease. But Hahnemann refused to throw the baby out with the bath water, and so found a way to prepare medicines – - through the process of potentization - – to optimize their curative effects, and diminish their toxic effects.
Hahnemann’s observation of the dual action of medicine provides the rationale for his determination that the simile principle is curative while the principle of opposites only palliates or suppresses.
He observed that most medicines have more than one action; the first is a direct action. This gradually changes into a second, indirect action, which is generally a state exactly the opposite of the former. If a physician gives a substance that has a direct action opposite to the natural disease, this is followed by the indirect action, which is similar to the disease. Thus palliative remedies, applied in accordance to the law of opposites, do more harm than good: the initially antagonistic action is followed by a secondary action, which is similar to the disease itself.
If a physician gives a substance whose direct primary action corresponds to the disease, the indirect secondary action is sometimes exactly the state of body sought to be brought about. Hence the law of similars cures disease rather than suppressing it.