It's said to cure women's low libido by making sex more pleasurable. But with fears over side-effects, many doctors are sceptical.
After the birth of her second child, Hilary Trent lost all interest in sex. In truth, the 45-year-old NHS administrator, from North London, had never enjoyed a particularly passionate relationship with her husband, Matthew. "The day we got married, we didn't have sex - we ate pizza," she recalls.
She and Matthew had known each other for many years before they became a couple in their 30s. "The sex was quite good for a while after we got married, though quite infrequent, even during the early days," says Hilary.
While she's enjoyed much more passionate sex in previous relationships, Hilary thought a physically cooler marriage would not be a problem. "We settled down to having sex around once a week - though, frankly, it was always a bit of an effort," she says.
Two years after getting married, the couple decided to try for a baby. Hilary quickly became pregnant. "We stopped having sex after I gave birth," she says. "My sex drive had gone. But, after a while, I thought that we should resume having sex, or it would threaten the marriage."
Having a child only made things worse. "It turned out that my husband was not happy about being a father," says Hilary. "There was tension and anxiety between us as a couple. It just further impacted on my sex drive.
"I wanted a second baby, but the sex became so mechanistic, it killed it for us both. After conceiving successfully for a second time, that was it. We almost never made love again.'
Early types of 'pink' Viagra failed
Loss of sexual relationships after childbirth is a common problem. A study of 1,500 new mothers by the Murdoch Children's Research Institute in Australia found that "almost all the couples interviewed had sex less often after having a baby".
But Hilary's sex life had become non-existent.
"Friends reassured me that my libido would come back, but it never did in that relationship," she says.
She turned to self-help books and tried talking to her husband about the problem. "But whenever I tried to instigate a conversation - which I found terribly embarrassing anyway - he would leave the room."
What she needed, or what the pharmaceutical companies would claim she needed, was a libido-boosting pill.
Over the years, there have been many attempts to create such a pill. Not least because a 'pink Viagra' promises to be a sure-fire money-spinner for the manufacturers if the success of Viagra is anything to go by; since 2003, worldwide sales of Viagra, made by drugs giant Pfizer, have reached around £1.2 billion a year.
However, so far, all attempts to create a female version have failed.
Male impotence is often caused by problems with lack of blood flow to the genitals; Viagra works by improving blood flow.
In 2004, Pfizer tested Viagra on women. But while it did increase blood flow, it didn't increase their arousal or result in them having sex more regularly.
Another approach involved giving women the male sex hormone testosterone to boost desire.
While in some reported cases this helped, gels and patches can have unfortunate side-effects, such as body hair and other male characteristics, and there are also fears they may be linked to blood clots.
A testosterone patch called Intrinsa was available on the NHS from 2007, but in 2012 the manufacturer cancelled its licence, citing commercial reasons.
In 2011, a testosterone ointment, LibiGel, was found to be no more effective than a placebo, so wasn't given a licence.
There are other lower-dose patches, but these are not licensed for women in the UK (although a doctor may agree to prescribe them 'off-label').
New drug works on the brain
But now, the first drug for treating low sexual desire in women is set to go on sale.
Earlier this month, a U.S. Food and Drug Administration (FDA) advisory panel voted to give conditional approval to a drug called Flibanserin, which will be marketed under the brand-name Addyi.
Final approval is expected in August, which may then pave the way for it to be prescribed in the UK.
The drug is controversial, however, not least because of its side-effects, such as sleepiness, sudden drops in blood pressure and fainting, especially in combination with alcohol.
In clinical trials, the side-effects were enough to make one in six women stop taking the medication.
Addyi does not act directly on the genitals, but works on women's brains, changing the way they respond to 'feelgood' chemicals such as serotonin and dopamine. The theory is that this may make their brains experience more pleasure during sex, and that the raised levels of these 'reward' chemicals may also reinforce pleasurable memories of the encounters, making women more likely to want to repeat the experience.
Worries about side-effects
It is not fully understood which brain mechanisms may be involved in low female libido. But a 2009 study from Stanford University School of Medicine in the U.S. identified a potential piece of the jigsaw.
MRI scans showed that women with low libido had lower than normal activity in the entorhinal cortex, an area of the brain involved in storing positive emotional memories.
This suggests that women suffering from low desire may not store memories from enjoyable sexual events and, therefore, cannot later retrieve those memories to motivate themselves to have more sex, according to the lead researcher, Dr Leah Millheiser, director of the Female Sexual Medicine Program. "It may not be the whole story, but it was a significant difference," she says.
But sceptics say Addyi is just a "failed antidepressant".
Indeed, it was originally developed as an antidepressant drug. But when it failed in clinical trials for depression, the drug's then owner, Boehringer Ingelheim, reinvented it as a potential treatment for low libido in women.
In 2010, the FDA unanimously rejected Boehringer's application to have it approved as a sex-boosting drug because it was worried about the side-effects. It was rejected for the same reason in 2013; by then, the drug's rights had been sold to Sprout Pharmaceuticals.
This month, Sprout's persistence finally paid off, when new safety data presented to the FDA indicated that the side-effects may not cause women lasting damage. However, to maintain the brain-altering effect from Addyi, women have to take the drug every day - whether or not they want sex.
But most tellingly, for all its potential side-effects, the drug doesn't seem terribly impressive. In tests, couples who had been having sex, or other 'sexually satisfying encounters' between two and three times a month, had an average of one additional sexual encounter a month when the woman took Addyi every day.
Nevertheless, Sprout has lobbied strongly, arguing that the drug's problems with gaining approval amounted to sexism. It said that there are several medicines available for treating male impotence, yet none for women with low desire. It even enlisted women's groups to support its argument.
Women's sex drive fluctuates
The 'disorder' that Addyi targets is labelled hypoactive sexual desire disorder - defined as the deficiency or absence of sexual fantasies and desire for sexual activity, causing significant personal distress or difficulty in relationships.
An estimated one in ten women suffers from it, according to Sprout Pharmaceuticals. But the drug targets "a desire disorder that doesn't exist", according to Ray Moynihan, an Australian medical researcher and leading campaigner against the medicalisation of normal human behaviour.
"Approval of Addyi could see widespread overprescribing of a drug with marginal benefits and real safety concerns," argues Moynihan, who is a senior health researcher at Bond University in Australia.
In fact, 'hypoactive sexual desire disorder' has been deleted from the psychiatrists' bible, the Diagnostic and Statistical Manual of Mental Disorders (DSM). In the latest version, it's been replaced with a new definition called 'sexual interest/arousal disorder'.
This new definition "is designed to more accurately reflect the complexity of women's sexual experience," says Moynihan. It recognises the fact that a woman's sex drive very often naturally ebbs and flows in harness with their emotional state and with the quality of their relationship.
The change in definition was based on a review of the scientific literature which concluded that a lack of spontaneous desire may be normal for the majority of women, many of whom still very much enjoyed their sex lives, and that this should not be seen as abnormal.
"With much stricter diagnostic criteria, the new condition's prevalence is likely to be a fraction of the 10 per cent claimed for hypoactive sexual desire disorder," adds Moynihan.
Tapping into women's memories
Other drug companies are also now trying to develop sex-drive drugs that affect women's brains.
Palatin Technologies is launching a human trial of bremelanotide, a synthetic hormone that acts on the hypothalamus, an area of the brain associated with memory and emotion.
The drug activates a receptor thought to make a woman sexually turned on by external stimuli, for example, their partner's voice. Such attempts, however, ignore the fact that women's sex drives are often determined by their emotional lives, according to Janice Hiller, a consultant clinical psychologist at London's Tavistock Centre for Couple Relationships.
"Women are different from men. The link between the brain and the genitals is different," she argues.
"The problem of waning sex drives for the majority of women is invariably about the quality of their relationships," she explains.
"Research has shown that if women feel belittled, ignored, taken for granted, undermined or subjected to hostility, then that will reduce or take away any sexual interest they have in their partner."
She is critical of Sprout Pharmaceuticals' tactics in seeking approval of Addyi.
"The research was done on a small group of women who fit the particular criteria for a diagnosis called hypoactive sexual desire disorder. This is a label that I wouldn't like to use, even on this small group of women," she explains. "This medicalises the problem as much as possible, because the women in the drug trials must not have any emotional factors, such as being in a state of anxiety. This makes them a much narrower group of women than the usual clients that we see presenting with low-libido problems in Britain."
Instead, Hiller argues that helping women understand why their sex drives are low is the key to tackling the problem.
"We encourage people to work on their relationships," she says.
"Sometimes women will come along with a lot of relationship dissatisfaction and then say that they don't know why they don't want to have sex any more. But really, they've just explained it without noticing."
The key is empathy, not libido
Ammanda Major, a Relate counsellor and sex therapist, echoes Hiller's sentiments, and adds that the stress of bringing up a family or caring for elderly parents can also reduce a woman's sex drive. "When life is overtaking a woman with stress and anxiety, the way that she feels about her partner sexually is enhanced by feeling that she has got his support."
This is how Hilary Trent feels with the benefit of hindsight about her loss of libido in her marriage. "It was not solely a sex issue, it was a relationship issue," she says. "There was no real chemistry between us, I now realise.
"Women just don't function the same way as men - that if all the right buttons are pressed, then it will work."
After Hilary split from her husband, she was prescribed antidepressants for two years.
"The drugs only continued to kill my libido. But after I came off them, I found myself in a new relationship where my libido returned - and the sex is so much better than before," she says.
"The difference, I believe, is that my new partner and I have a real emotional connection. The chemistry is there. But, just as important, I feel emotionally secure. I don't think a pill could ever create that."
Hilary Trent and her former husband's names have been changed.
- Daily Mail