Postexposure conditioning, as a part of hormesis, involves the application of a low dose of stress following exposure to a severe stress condition. The beneficial effect of a low level of stress in postconditioning hormesis is illustrated by a number of examples found in experimental and clinical research. Depending on whether the low-dose stress is of the same type of stress or is different from the initial high-dose stress causing the diseased state, postconditioning is classified as homologous or heterologous, respectively. In clinical homeopathy, where substances are applied according to the Similia principle, the same distinction is found between the isopathic and the 'heteropathic' or homeopathic use of low dose substances. The Similia principle implies that substances causing symptoms in healthy biological systems can be used to treat similar symptoms in diseased biological systems. Only when heterologous substances are tested for therapeutic effects, the Similia principle can be studied. It is then possible to compare the effect of treatment with the degree of similarity between the diseased state and the effects caused by different substances. The latter research was mainly performed with cells in culture using heat shocked cells post exposed to a variety of stress conditions in low dose.