According to what may be legend, King Gustav III of Sweden conducted that country's first clinical trial during the second half of the 18th century. He wanted to determine whether drinking coffee was bad for one's health. He firmly believed it was. The story is told on the website of Sweden's Uppsala University Library:
The king Gustav III viewed coffee consumption as a threat to the public health and was determined to prove its negative effects. It is said that he decided to carry out an experiment on two prisoners. Two twins had been tried for the crimes they had committed and condemned to death. Their sentences were commuted to life imprisonment on the condition that one of the twins drank three pots of coffee every day while the other drank the same amount of tea, and this for the rest of their lives, in order to see if the coffee affected their life expectancy. Unfortunately the king died before the final result of his experiment: the first twin died at the age of 83 and he was the one who drank tea!
Barstow Desert Dispatch - Jan 7, 1991
that the authenticity of the coffee experiment story has been questioned. Though it doesn't say why.
As far as I can tell, the earliest English-language reference to the story appeared in a 1937 issue of The Science News-Letter
. This account was then widely reprinted in newspapers (see below).
The Science News-Letter
attributed the information to the Swedish-born botanist Bror Eric Dahlgren, who was a curator at the Field Museum in Chicago. Dahlgren did author a 1938 pamphlet about the history of coffee, which you can read online at the Biodiversity Heritage Library
, but it doesn't include the story of King Gustav. I can't locate where else Dahlgren might have told the story of the coffee experiment, which makes it impossible to check his references.
The Sheboygan Press - May 28, 1938