One of the many things he is known for is his teaching that all sub-
stances are poison and that whether something acts as a poison or a
medicine may depend on the dose.
Everything is a poison. Even substances essential for human life,
such as water and oxygen, are poisons if taken in sufficiently large
amounts. And all chemical substances that are taken as medicines—
that may have beneficial therapeutic effects on the body—are also,
and at the same time, poisons. This is a very powerful teaching.
Although everything may be a poison, some things are more poi-
sonous than others. One of the most poisonous substances known is
a chemical called tetrodotoxin (TTX). This poison famously occurs
in the body of certain varieties of puffer fish (also called blowfish
or fugu) that live throughout the world in tropical and semitropical
oceans. TTX has also been identified in other animals, including
several varieties of salamander or newt, a blue-ringed octopus, and
various crabs and starfish. That a weird molecule like TTX is found in
a number of unrelated animal species suggests it is not synthesized
directly by the animals themselves but, rather, may be the product of
a microorganism living inside these various animals. Indeed, TTX is
now known to be made by bacteria living symbiotically within the an-
imal species found to possess it. The bacteria get a nice place to reside,
with food and shelter provided. And the animals harboring the TTX-
making bacteria receive protection—because of TTX’s poison qualities
—from being eaten by predators.