"Yes."

"Like earlier to bed?"

"Yes." Linneah's voice lurched louder; she laughed; it was a relief to talk bluntly.

"Do you have pleasant feelings when you're touched?"

"Yes."

Later, after her appointment, she told me that in fact she has orgasms pretty much every time she and her husband have sex — that wasn't the problem. "There's something that's stopping me from wanting it," she said. "I don't know what it is. I can't tell you what it is."

She met her husband at a bar nearly two decades ago: she joked with him over a foosball table, watched him clown on the dance floor. "I had a professor at college who talked about 'the attraction template.' My husband's right inside my attraction template," she said. She remembered his dark hair, his boyish looks, the way she'd felt they fit together, because they were both on the short side. "And he's a stand-up guy. He has an excellent sense of what's right and what's wrong. He can make fun but not in a way that hurts anyone."

When they were dating and out with other couples, Linneah would think, "I just want to get home with him, I just want to get home with him," she recalled. But that lust had dwindled. Around the arrival of their second child in 2004, something insidious crept in, partly fatigue but partly something else that she couldn't name. She talked about her to-do lists, the demands of the kids, "but let's face it," she said, "sex doesn't take that much time." Rather than feeling as if she still wanted to grab her husband's hand and hurry him up the stairs in their small brick house, on many nights she waited in bed, somewhat like prey, though the predator was tender, though he was cherished.

Around once a week, her husband tried to reach through the invisible barriers she built — the going up to bed early, the intense concentration on a book, the hoping he was too tired to want anything but sleep. "He'll move closer to me in bed, or put his arm around me, or rub my back." She willed herself not to refuse him. And mostly, she didn't. Usually they had sex about four times each month. But it upset her that she had to force herself and that she put up those barriers to deter him from reaching more often.

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"I'm scared that if it's slimmed to this by now, what's going to happen as we get older?" she said. "I want to stay close, not just psychologically, physically. I want to stay in love. I have a friend, they have sex so intermittently, every three months. She is so unhappy. I don't want that to happen to me." She longed for a cure, a tab of magic. As she got into her car in the parking lot at the center, she hoped that her first set of pills had been placebos, that she'd been given fakes for the first eight weeks, that today she was driving away with the real drug and that their sex life would be transformed.

Half a century ago, the birth-control pill offered women the ability to switch off ovulation, to separate sex from reproduction. It played a part, as the '60s got under way, in propelling a host of profound changes, cultural as well as reproductive, societal as well as intimate — in how women saw themselves and lived their lives, starting with the notion of women being above all baby makers and mothers. The promise of Lybrido and of a similar medication called Lybridos, which Tuiten also has in trials, or of whatever chemical finally wins the race for F.D.A. approval, is that it will be possible to take a next step, to give women the power to switch on lust, to free desire from the obstacles that get in its way. "Female Viagra" is the way drugs like Lybrido and Lybridos tend to be discussed. But this is a misconception. Viagra meddles with the arteries; it causes physical shifts that allow the penis to rise. A female-desire drug would be something else. It would adjust the primal and executive regions of the brain. It would reach into the psyche.

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Beckoned by ads on the radio and in newspapers and on Craigslist, in the fall of 2011 women across America began applying to be among the 420 subjects in the Lybrido and Lybridos studies. Plenty were turned away when the trials filled. Lack of lust, when it creates emotional distress, meets the psychiatric profession's clinical criteria for H.S.D.D., or hypoactive sexual-desire disorder. Researchers have set its prevalence among women between the ages