This is 6-n-propylthiouracil, otherwise known as PROP. It once had a conventional medical purpose. Now it's being shoved into people's mouths to see if they're genetically gifted (or cursed) with the power of supertaste.
People first were dosed with 6-n-propylthiouracil, or PROP, when they discovered they had hyperthyroidism. The drug took out an enzyme required to make thyroid hormone, and so shut down its overproduction in the thyroid. Over time, doctors discovered that it could cause liver damage, and so it fell out of favor. By that time, however, doctors discovered another use for it.
Some patients found the drug to be unbearably bitter. Some found it only slightly bitter. Some didn't notice a flavor at all. With a little research, scientists found that those who could taste the bitterness in PROP tended to have more taste receptors, and that this ability seemed to run in families. Now PROP is used in one of tests that determines if someone is a supertaster. Patients swish a cup of liquid, with some PROP mixed in, around in their mouths, or they put a paper saturated with PROP on their tongue. If they're supertasters, they won't want to be tasting long. Supertasters find PROP overwhelmingly bitter and unpleasant. People with slightly heightened senses of taste find it only slightly bitter. Everyone else tastes only the water or the paper.
Supertasters aren't only repulsed by the bitterness in PROP. They can also taste the bitterness in alcohol, and in caffeinated drinks. Adding a lot of sugar to the drinks can mitigate the taste, and supertasters, like everyone else, love the taste of sugared-up coffees. Still, for the most part, people who respond badly to PROP tend to be the sort of people who only go into coffee shops for the pastries, and sit hollowed-eyed in bars, wondering why they never serve cake. Life's bitter enough already.
An interesting side note - PROP is the first test for a supertaster. The next one? A peppermint LifeSaver. If Wrigley announced on the wrapper that peppermint LifeSavers were medical-strength candy, I'd start buying them.