To quote from wikipedia:

“Denatonium, usually available as denatonium benzoate (under trade names such as BITTER+PLUS, Bitrex or Aversion) and as denatonium saccharide, is the most bitter chemical compound known, with bitterness thresholds of 0.05 ppm for the benzoate and 0.01 ppm for the saccharide.^[2] It was discovered in 1958 during research on local anesthetics by MacFarlan Smith of Edinburgh, Scotland, and registered under the trademark Bitrex.^[3] Dilutions of as little as 10 ppm are unbearably bitter to most humans. Denatonium salts are usually colorless and odorless solids but are often traded as solutions. They are used as aversive agents (bitterants) to prevent accidental ingestion. Denatonium is used in denatured alcohol,^[4] antifreeze, nail biting preventions, respirator mask fit-testing, animal repellents, liquid soaps, and shampoos. It is not known to pose any long-term health risks.^[5]“