The drug Viagra revolutionised the treatment of sexual dysfunction in men on its launch five years ago. An accidental discovery, the tablet that gave impotent men the chance once more to have natural erections became the fastest selling pill in history and has earned its manufacturer, Pfizer, over $6bn. The search is now on for a similar drug that could help women. Research is revealing that female sexuality is more complex than expected. For women suffering from a loss of desire many scientists believe that drugs acting on the brain may be the way forward. A pioneering Scottish study may have identified just such a drug and begun testing it scientifically. An erection is achieved by filling the erectile tissue of the penis with blood. Blood vessels widen to allow blood in and then constrict to maintain the pressure. Male impotence was long thought to be a psychiatric effect, a result of stress, anxiety or depression. Medical advice was that there was not much to be done. Some patients refused to take this message on board.