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The tablet was patented in 1996, approved for apply in erectile dysfunction by the FDA on March 27, 1998, becoming the first medicine approved to treat erectile dysfunction in the United States, and offered for sale in the United States later that year (Kling 1998). It soon became a great success: annual sales of Viagra in the period 1999-2001 exceeded $1 billion. Pfizer therefore decided to market it for erectile dysfunction, rather than for angina. Phase I clinical trials under the direction of Ian Osterloh suggested that the medication had little result on angina, but that it may induce marked penile erections (Boolell et al 1996). It was initially studied for operate in hypertension (high blood pressure) and angina pectoris (a shape of cardiovascular disease). Sildenafil (compound UK-92,480) was synthesized by a group of pharmaceutical chemists working at Pfizer's Sandwich, Kent research facility.