Answer: Denatonium

The human sense of taste is quite refined and there are all manner of bitter flavors in the world that we often incorporate into our diets for novelty, like packing sour candy with citric acid, or go out of our way to mask, like when British soldiers created the gin and tonic drink in order to mask the bitter taste of the anti-malarial quinine they had to consume.

But when it comes to bitter compounds, however, the bitterness of things like citric acid and quinine have nothing on denatonium. Discovered accidentally during anesthetic research conducted by pharmaceutical company MacFarlan Smith in the 1950s (the chemical structure of it is very similar to lidocaine, a widely used local anesthetic), it proved more bitter than any known compound. It is so bitter that a dilution of 0.01 parts per million (the saccharide form) to 0.05 parts per million (the benzoate form) is detectable to humans and 10 parts per million is unbearably bitter to the point that the consumer of a liquid laced with even a trace amount of denatonium would scarcely be able to even swallow the liquid in question.

And that right there, lacing liquids, is exactly how denatonium, under the trade name “Bitrex”, finally became a widely used chemical after languishing unused for years after its discovery. After a rise in children being hospitalized after ingesting household chemicals in the 1980s, some companies started including a tiny amount of Bitrex in their products to deter accidental consumption. Still, despite widespread use in Europe, very few U.S. companies used it. A grass roots campaign by a woman named Lynn Tylczak caught the attention of U.S. media which, in turn, led to much higher adoption rates.

Today denatonium/Bitrex is found in thousands of products ranging from harsh household cleaning products to shampoos and soaps to antifreeze, all in a bid to keep children and animals from ingesting dangerous chemicals. In such cases, an ounce (perhaps more of an attogram) of prevention is worth a pound of cure.