Samuel Hahnemann attributed fundamental importance to the principle of similitude, promoting it to a 'natural law'. Observing that enantiopathic or allopathic treatment produced enduring aggravation of the disease symptoms after a brief and transitory initial relief, he systematised homeopathic treatment, prescribing substances that provoke similar symptoms in healthy individual. Based on clinical and experimental observations, he anticipated the concept of homeostasis, dividing the effects of substances into: primary action of the medicine followed by secondary action or reaction of the organism. This reaction, known as the rebound effect or paradoxical action by modern pharmacology, used to awake the curative response of the body when the principle of similitude is applied, is responsible for several iatrogenic diseases when used on the basis of the principle of contraries. This study discusses the role of this paradoxical reaction of the organism in the fatal side effects of four important drugs, used according to the model of enantiopathic treatment of the symptoms. I present evidence relating to acetylsalicylic acid, rofecoxib, antidepressants and long-acting bronchodilators. The consequences of the allopathic treatment could be decreased if health professionals valued homeostasis, minimising the rebound effect of the organism by gradual suspension of palliative drugs.