Secretin
Secretin was the first hormone to be identified. In 1902, William Bayliss and Ernest Starling were studying how the nervous system controls the process of digestion. It was known that the pancreas secreted digestive juices in response to the passage of food (chyme) through the pyloric sphincter into the duodenum. They discovered (by cutting all the nerves to the pancreas in their experimental animals) that this process was not, in fact, governed by the nervous system. They determined that a substance secreted by the intestinal lining stimulates the pancreas after being transported via the bloodstream. They named this intestinal secretion "secretin". Secretin was the first such "chemical messenger" identified. This type of substance is now called a hormone, a term coined by Bayliss in 1905.
Opium
Heroin, the first semi-synthetic opioid, was first synthesized in 1874, but was not pursued until its rediscovery in 1897 by Felix Hoffmann at the Bayer pharmaceutical company in Elberfeld, Germany. From 1898 to 1910 heroin was marketed as a non-addictive morphine substitute and cough medicine for children. Because the lethal dose of heroin was viewed as a hundred times greater than its effective dose, heroin was advertised as a safer alternative to other opioids. By 1902, sales made up 5 percent of the company's profits, and "heroinism" had attracted media attention. Oxycodone, a thebaine derivative similar to codeine, was introduced by Bayer in 1916 and promoted as a less-addictive analgesic. Preparations of the drug such as oxycodone with paracetamol and extended release oxycodone remain popular to this day.