Neil Sweig, an analyst at Fulcrum Global Partners, who emphasizes that Viagra was still a "breakthrough product, " points out two other problems: Viagra is widely counterfeited, particularly outside the U.S., and the pill doesn't work for one in three men who try it.

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Mr Pope told me that he has not prescribed Viagra on the NHS, but has used it for a handful of private patients with about a two-thirds success rate.

BBC: News | Health | Viagra: a great British invention