In 1991, Pfizer scientists in Sandwich, England, synthesised a compound called UK-92,480 — later named sildenafil — as a treatment for hypertension and angina pectoris (chest pain from heart disease). The drug worked by inhibiting an enzyme called PDE5, which relaxes blood vessels. The theory was sound: more relaxed vessels meant lower blood pressure and less strain on the heart.
Phase I clinical trials in 1994, led by Dr Ian Osterloh, showed that sildenafil had almost no effect on angina. But the male volunteers reported an unexpected side effect: sustained erections. The trial site in Merthyr Tydfil, Wales, became famous for the number of men who refused to return their unused pills. Pfizer faced a choice — abandon the compound or pivot to a condition the company had never marketed before.
Pfizer pivoted. Sildenafil was patented in 1996, approved by the FDA on 27 March 1998, and launched later that year as Viagra. It was the first oral treatment for erectile dysfunction approved in the US. Annual sales peaked at $1.934 billion in 2008, and by 2000 Viagra held 92% of the global ED prescription market. A failed angina drug became one of the most commercially successful accidental discoveries in pharmaceutical history.
Pfizer pivoted to the side effect—erections—repurposing the failed heart drug as Viagra, the first oral ED treatment, which peaked at $1.934 billion in annual sales.
Pfizer spent years developing sildenafil as a heart drug that simply did not work for its intended purpose — the mechanism was right, the application was wrong
The clinical trial in Wales revealed the side effect, but Pfizer had to decide whether to invest in a condition the company had no experience marketing — a risky bet on an unproven market
The pivot required regulatory approval for a new indication, clinical trials with a sensitive endpoint, and a marketing campaign navigating social taboos around erectile dysfunction
Despite the risks, Viagra became the fastest-selling drug in history at launch, reaching $1.934 billion in peak annual sales and holding 92% of the global market by 2000
A drug that fails its primary purpose is not a failed drug — it is a drug whose purpose has not been found yet. Pfizer's pivot from angina to ED turned a disappointment into a $1.9B blockbuster.
FOLLOW THE EVIDENCE