While his partner may feel ignored, a man's lack of sexual desire may be due to a chemical imbalance in his brain that goes undetected.

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While hanging out with his college roommates, Peter (not his real name) realized he felt differently about sex than other heterosexual men.

“I’ve never been somebody who was interested in pornography, but I’d laugh along with their jokes,” said Peter, now 44, who is British. “Of course I never mentioned that … as a man, you’d be kicked out of the herd.”

As he developed “proper, serious relationships” with women, Peter discovered he didn’t have the sexual drive many of his partners did.

“I would make excuses around getting tired or feeling stressed, that kind of thing,” he said. “It wasn’t an issue with attraction to my partner. It just didn’t enter my mind to initiate sex.”

In 2021, Peter saw an ad recruiting male volunteers for a new study on hypoactive sexual desire disorder, or HSDD. Researchers planned to inject the study’s participants with kisspeptin —a naturally occurring sexual hormone — to see if it increased their sex drive. Kisspeptin plays a key role in reproduction; without adequate levels of the hormone children do not go through puberty, for example.

In a long-term, committed relationship with a woman he says has a higher sexual appetite, Peter signed up, intrigued by the thought that a biological imbalance might help explain his behavior.

In the week after the final session, Peter said, something amazing occurred.

“All of a sudden, I wanted to initiate intimacy. I can only presume it was driven not by my mind remembering something, but my body wanting something,” he said. “I did initiate sex more and it improved things with my partner incredibly.”

Wiring in the brain

Experts believe HSDD affects at least 10% of women and up to 8% of men, although those numbers may be low, said Stanley Althof, a professor emeritus of psychology at Case Western Reserve University School of Medicine in Cleveland, Ohio and executive director of the Center for Marital and Sexual Health of South Florida.

“Men are embarrassed to go to the doctor to begin with, and you’re supposed to be a macho guy,” said Althof, who was not involved in the kisspeptin study.

“So it’s difficult for men to say, ‘Hey, I’ve got a problem with my sex drive.’ That’s why the majority of male patients I see with HSDD are sent in by their partners.”

To be diagnosed with the disorder, a person must have no other issues that might cause a change in libido, such as erectile dysfunction or premature ejaculation.

“Losing interest due to performance issues is common, but HSDD is its own thing,” Althof said. “It’s an absence of erotic thoughts and a lack of desire for sex that has to be present for six months. It also cannot be better explained by another disorder or other stressors: It can’t be due to depression. It can’t be due to a bad relationship. It can’t be due to taking an antidepressant.”

One more key point: A man or woman must have clinically significant distress to have HSDD, said clinical psychologist Dr. Sheryl Kingsberg, a professor in reproductive biology and psychiatry at Case Western Reserve University, who was also not involved in the kisspeptin study.

“Some people aren’t bothered by their lack of interest in sex, so we wouldn’t treat them for HSDD,” said Kingsberg, who is also chief of behavioral medicine at MacDonald Women’s Hospital and University Hospitals Cleveland Medical Center.

“The women coming into my office are deeply distressed,” she said. “They tell me ‘I used to have desire but it’s gone. I could be on a desert island with no pressures, but I just don’t have the appetite. I want it back.’ Those women have HSDD.”

‘Surreal’ experience

Dr. Waljit Dhillo, a professor in endocrinology and metabolism at Imperial College London, has been studying the relationship between low sexual desire and the hormone kisspeptin for years, first in animals, then in people.

Prior studies by Dhillo of healthy men with no libido problems found giving them kisspeptin boosted levels of testosterone and luteinizing hormone, which is important for gonad function.

His newest study, published in the journal JAMA Network Open in February, enrolled 32 men with verified HSDD. Peter was one of them.

“So many people say to themselves, ‘It’s just me. I’ve got a problem.’ But actually, HSDD may be how your brain is wired,” said Dhillo, who is a dean at the United Kingdom’s National Institute for Health and Care Research Academy in Newcastle upon Tyne.

“The biology is telling us there’s increased activation of inhibitory areas in the brain — the same areas that tell us it’s not OK to walk around in public naked — and those areas are switching off sexual desire. How can we tackle that? We give a hormone that would naturally give you increased sexual desire, essentially hijacking the normal system.”

The men participating in the new study visited Dhillo’s lab twice. On each occasion, they were fitted with a device to objectively measure arousal, given an injection and asked to watch pornography while their brains were scanned via functional magnetic resonance imaging (fMRI).

Neither the subjects or the researchers knew if that day’s injection was kisspeptin or a placebo.

“It was extraordinarily surreal, lying there with something resembling a hangman’s noose around your bits and watching a mixture of ’70s to modern-day pornographic images and videos,” Peter said. “You’d get about five or six seconds of one type of image or video, rate your arousal for the researchers, and then move on to the next.”

Brain scans showed a significant dual effect after the kisspeptin injection, Dhillo said. Activity in the areas of the brain that inhibit behavior slowed, while areas of the brain connected to sexual interest lit up.

“As a group, the men had a 56% higher sexual response to sexual images after the kisspeptin than the placebo,” Dhillo said. “And we found no side effects at the very, very small dose that we are using.”