Viagra: the failed heart drug that became a blockbuster

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.