The simillimum (also spelled similimum) in homeopathic medicine is a single substance which if given to a healthy person would produce exactly the same pattern of symptoms as exhibited by a patient. If that substance is diluted and potentized according to homeopathic principles it would make a specific homeopathic remedy that would bring about the best cure in that individual.
The "Law of Similars" is a fundamental concept of homeopathic medicine, expressed by Samuel Hahnemann as similia similibus curentur, or 'let likes cure likes'. It has some similarities with the doctrine of signatures in herbal medicine.
The law of similars is viewed by some authors as a basic law of nature. () Analogies have been drawn to some aspects of orthodox medicine, where "the mode of functioning for some medicines probably can be ascribed [to] the law of similars." ()
A double-blinded, randomized, placebo-controlled clinical trial found that response to the simillium for sufferers of fibromyalgia may be linked with executive function at the prefrontal cortex, an area of the brain that controls the ability to redirect attention, as defined by prefrontal electroencephalographic alpha frequency cordance. Trait absorption (genetically determined ability to focus attention selectively and fully) was also found to correlate with response to the simillium. ()
There may be diverse interpretations of the law of similars amongst homeopaths in their choice of simillium, as suggested by research based on a paper case analysis questionnaire. ()
In common with many alternative therapies, the fundamentally holistic concept of the simillium has been criticized by pluralists for being monistic, ignoring any distinction between apparently independent parts of nature. ()