Viagra, the 'little blue pill' that is used by millions of men worldwide for erectile dysfunction, was actually discovered by accident.
In the early 1990s, researchers working at the pharmaceutical company Pfizer were trying to invent a treatment for angina, a type of chest pain caused by reduced blood flow to the heart.
One candidate was the drug sildenafil. By blocking a particular protein called phosphodiesterase type 5, also known simply as PDE5, the drug could cause blood vessels to dilate .
In a trial conducted in the early 1990s, men taking sildenafil reported side-effects such as flushing, headache, gastrointestinal effects, and nausea. They also reported having erections, which was completely unexpected.
"We were a young team in those days and erectile dysfunction wasn't something that concerned us, so we didn't really know there was a need for a drug like this," Sir Simon Campbell, an organic chemist who helped develop the drug, explained to BBC's Scientifically podcast.
At the end of the trial, some of the participants refused to return their pills, expressing a desire to continue taking them.
And so, the research team quickly pivoted towards testing sildenafil for erectile dysfunction instead of heart disease.
Sildenafil is now branded as Viagra, a pill more associated with affairs of the heart than cardiac function.
How does sildenafil treat erectile dysfunction?
Sildenafil works by stopping the breakdown of cyclic GMP in the smooth muscle cells in the blood vessels in the penis, which increases blood flow during sexual arousal. In people with erectile dysfunction, there isn't enough cyclic GMP.
The drug was a smash hit. Within weeks of being approved by the FDA in 1998, more than one million Viagra prescriptions were dispensed in the United States. The drug is now estimated to be taken by 62 million men worldwide, according to its manufacturer, Pfizer.
Later, clinical trials revealed that sildenafil could also be used to treat heart conditions such as pulmonary arterial hypertension after all, as it was originally intended.
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