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'Arndt-Schulz rule' or 'Schulz' law' is a law (named after Hugo Paul Friedrich Schulz and Rudolf Arndt) concerning the effects of pharmaca or poisons in low, respectively strong concentrations. According to this, highly diluted pharmaca or poisons enhance life processes, moderately strong ones favour it, while strong concentrations may inhibit these processes and even terminate them.
This rule has been applied intensively by homeopaths, to support their theories. However, the exceptions to the rule are so numerous that it can not be considered a general law.
For instance, many paralysing substances have no exciting effect in weak doses, and what constitutes a weak, medium or strong stimulus is highly individual, as pointed out by Arndt.
There is also the consideration that dilutions typically used by homeopaths contain none of the diluted substance with a high probability, which would mean this rule wouldn't apply. This objection led to the postulation of "water memory".