US regulators have approved a prescription drug to boost libido in women.

The new product, an injectable administered like an EpiPen, was given the green light on Friday to be prescribed to premenopausal women who struggle with sexual desire and distress.

It is the second drug approved to treat low sex drive in women; the first (Addyi, or 'the little pink pill') was approved in 2015, but was a commercial flop largely because women could not drink alcohol while using it.

In April, the US Food and Drug Administration ruled that women could drink on Addyi, and investors are holding their breath to see if that move triggers and uptick in demand and sales.

In the meantime, another drug has come up to compete: Vyleesi (the brand name for bremelanotide, made by AMAG Pharmaceuticals) which has proved effective in two trials with 1,200 women - and, crucially, comes with no lifestyle restrictions.

The approval on Friday was hailed by proponents as another breakthrough for women, who have waited almost three decades for an equivalent to Viagra.

Part of the issue with HSDD treatments is that the condition itself is highly subjective, relies on an initial self-diagnosis, followed by a process of elimination, with doctors trying everything before they conclude that a patient has HSDD (file image)

Like Addyi pills, bremelanotide targets brain receptors (though, different ones) to ease inhibition in women diagnosed with hypoactive sexual desire disorder (HSDD), who have low libido and high levels of distress when intimate.

While Viagra and Cialis increase blood flow to a man's penis, Addyi (the brand name for flibanserin) and bremelanotide target the brain with similar effects to an antidepressant.

Addyi boosts dopamine to gear up motivation and excitement, and dulls serotonin, the neurotransmitter which makes us self-conscious.

Bremelanotide targets melanocortin 4, which eases distress and boosts desire.

In trials, bremelanotide proved markedly effective at reducing distress in women with HSDD: by the end of the trial, most saw a one-point reduction in distress, measured on a scale of 1-4.

Desire was more marginal - going up an average of 0.6 points on a scale of 1.2-6.

The manufacturers say even a slight increase in desire is significant for women with HSDD.

Part of the issue with HSDD treatments is that the condition itself is highly subjective, relies on an initial self-diagnosis, followed by a process of elimination, with doctors trying everything before they conclude that a patient has HSDD.

That makes diagnoses few and far between, or contested, and it makes it hard for regulators to deliver a clear-cut verdict on how 'effective' or 'necessary' treatments might be.

The American Sexual Health Association, a 100-year-old nonprofit source of sexual health information, says HSDD affects a staggering one in 10 women - and ASHA CEO Lynn Barclay told DailyMail.com visits to the HSDD page on their website have been rocketing in recent years and months.

However, many health professionals question whether HSDD is real.

The FDA's verdict on Friday adds weight to the ASHA's fight to have HSDD recognized.