Paracelsus' Dictum, addressing human toxicology, forms the basis for both modern medicine and ecotoxicology. The Dictum can be paraphrased as ‘The right dose makes the poison.’ What this means in terms of ecotoxicology, is that for toxicity to occur, an organism must be exposed to a contaminant and at the correct concentration. Effectively, this Dictum also separates pollutants (toxicity occurs) from contaminants (toxicity does not occur). However, as presently constituted, the Dictum neither includes nor considers two critical modifying factors, for example in the sedimentary environment: bioavailability and realistic exposure scenarios. Too many scientists, regulators and stakeholders are, explicitly or implicitly, applying the Dictum directly, without modification. Such literal interpretations have resulted in incorrect assessment of, for instance, the risk of polyaromatic hydrocarbon phototoxicity; the relative importance of pore water, overlying water and dietary exposure routes; biological tolerance mechanisms (avoidance, acclimation, non-genetic adaptation, genetic adaptation, metabolism); and secondary (indirect) toxicity. To ensure that the emphasis is on pollutants, not contaminants, the Dictum needs to be rephrased as follows and adhered to: ‘All substances are poisons; there is none which is not a poison. The right dose of a bioavailable substance, administered under realistic exposure conditions, differentiates a poison.’
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