Chemistry - What's the usual agent added to discourage drinking of hazardous substances
It is denatonium benzoate. This comes from wikipedia page of denatonium:
Denatonium, usually available as denatonium benzoate (under trade names such as Denatrol, BITTERANT-b, BITTER+PLUS, Bitrex or Aversion) and as denatonium saccharide (BITTERANT-s), is the most bitter chemical compound known, with bitterness thresholds of 0.05 ppm for the benzoate and 0.01 ppm for the saccharide. It was discovered in 1958 during research on local anesthetics by MacFarlan Smith of Edinburgh, Scotland, and registered under the trademark Bitrex.
With respect to discouraging drinking of hazardous substances it again comes from wiki:
The bitterness of the compound guides most applications of denatonium. Denatonium benzoate is used to denature ethanol so that it is not treated as an alcoholic beverage with respect to taxation and sales restrictions.
Many compounds are added to dangerous substances that might be accidentally or deliberately ingested
There isn't just one, universal, answer to this question.
The problem is that there are a variety of different hazards and specific countermeasures may be required for each. And ingestion of some dangerous materials occurs deliberately not just accidentally and providers need to act to minimise those risks as well as the accidental ones.
Take the classic example of the weedkiller paraquat. It took a while after it was first introduced for people to notice its toxicity, possibly after accidental ingestion of the solution which was often stored by domestic users in old bottles previously used for soft drinks. It is fairly deadly if ingested and allowed to get into the bloodstream.
Initial solutions were to add material such as Bitrex
which is so bitter it should provide adequate warning that the solution should not be drunk. But, once the deadly nature became public knowledge, many people deliberately took paraquat as a method of suicide. This prompted the addition of other ingredients designed to minimise the hazard even when taken deliberately.
The first key step was to add an emetic (designed to encourage the user to vomit, reducing the volume of the solution that can be absorbed in the stomach.) FAO agricultural standards actually specify the nature an emetic requires for paraquat though only one compound met the standard:
The compound known as PP-796 or 2-Amino-5-methyl-7-propyl-7H-1,3,3a,7-tetraazainden-6-one is a powerful, fast acting emetic.
Even this, though, is not fully effective and the manufacturers have explored a variety of other strategies including adding a natural alginate to the formulation that gels in the acid environment of the stomach slowing the absorption into the body.
While all these strategies have been applied to the particularly hazardous compound paraquat, they are representative of the other steps manufacturers will take to minimize the risk of accidental or deliberate use of their products.