Gustav III of Sweden's coffee experiment was a twin study ordered by the king to study the health effects of coffee. Although the authenticity of the event has been questioned, the experiment, which was conducted in the second half of the 18th century, failed to prove that coffee was a dangerous beverage.
Coffee first arrived in Sweden around 1674, but was little used until the turn of the 18th century when it became fashionable among the wealthy. In 1746, a royal edict was issued against coffee and tea due to "the misuse and excesses of tea and coffee drinking". Heavy taxes were levied on consumption, and failure to pay the tax on the substance resulted in fines and confiscation of cups and dishes. Later, coffee was banned completely; despite the ban, consumption continued.
The king ordered the experiment to be conducted using two identical twins. Both of the 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 drink three pots of coffee, and the other drink the same amount of tea, every day for the rest of their lives.
Two physicians were appointed to supervise the experiment and report its finding to the king. Unfortunately, both doctors died, presumably of natural causes, before the experiment was completed. Gustav III, who was assassinated in 1792, also died before seeing the final results. Of the twins, the tea drinker was the first to die, at age 83; the date of death of the surviving coffee drinker is unknown.
In 1794, the government once again tried to impose a ban on coffee. The ban, which was renewed multiple times until the 1820s, was never successful in stamping out coffee-drinking. Once the ban was lifted, coffee became a dominant beverage in Sweden, which since has been one of the countries with the highest coffee consumption per capita in the world.