Drug used to treat impotence,
approved by the US Food and Drug Administration (FDA) in March 1998. Viagra works by blocking the enzyme phosphodiesterase, which breaks down cGMP, a relaxant of smooth muscle. The cGMP allows for increased dilation of blood vessels (dilated by nitric oxide) and it is the increase in blood flow that enables two thirds of men suffering from erectile dysfunction to achieve an erection. It must be taken about an hour before intercourse. Side effects include headaches and fainting (due to dilation of blood vessels), and blue-tinted vision. The inhibition of phophodiesterase in the eye may result in damage to the retina as a side effect of taking the drug.
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By the end of 1998, the first year of its release, 5 million prescriptions for Viagra had been written (3 million in the first 14 weeks of its US release). At that time the US FDA announced that Viagra labels must list more side effects and a carry a safety warning stressing that the drug has not been proved safe for men with heart problems, following the deaths of 130 users in the USA since the drug's issue. Nevertheless, Viagra had the most successful first year of any drug ever launched. It reached $1 billion in sales on the first anniversary of its launch. By the end of 1998, sales of the pills, costing $7 each, amounted to $656 million in the USA alone; the total, including international sales, was $788 million. In 2005, it was estimated that 23 million men had received Viagra on prescription, with global annual sales amounting to (dollar)1.6 billion.
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Several studies have tested the drug for sexual dysfunction in women, but results are contradictory.
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