IT AFFECTS women as well as men, and it starts young. It's a secret that's ruining more and more lives.
A few months before their wedding, David (not his real name) told his fiancee Sue his biggest secret.
Although the couple had a good sex life, and were committed to a future together, he was addicted to visiting prostitutes.
Perhaps surprisingly, the wedding went ahead.
"I was horribly shocked, but I thought that with some therapy, he'd get over it," Sue, 42, said.
Fifteen years and two children on, the couple are still together - but David's sex addiction, too, is with him still.
"I don't think we could possibly have imagined that it would be as long term, or as difficult, as it has been," Sue said.
David and Sue are clients of a sexual psychotherapist, Paula Hall, who last month published the UK's first comprehensive guide to what sex addiction is.
She defines it as "a pattern of out-of-control sexual behaviour that causes problems in someone's life" - and how its sufferers can be helped.
No one knows how many sex addicts there are in Britain but, Ms Hall says, in her professional judgment, it's hugely on the increase.
No one understands what the rise in sex addiction is entirely about but internet porn, Ms Hall says, has got to be part of it.
"Porn is like the gateway drug," she said.
"Just as with cannabis and cocaine, many people will use the gateway drug and never become addicted.
"But others most definitely will."
The easy availability of porn gives people in high-stress jobs a way of dealing with their pressurised life-style.
In researching her book, Ms Hall surveyed 350 people who described themselves as addicted to sex, 25% of whom were women.
"The proportion of women addicts surprised me," she said.
"And what surprised me even more was the number who are using porn: 90% of the men I surveyed, and 74% of the women, said they were heavy porn-users."
The biggest problem for sex addicts, Ms Hall says, is that it's seen as a moral deficiency rather than a mental illness.
Also, despite the increase in people seeking therapy, most of those affected still try to deal with it alone - and the fallout can be devastating.
"People had lost their families, they'd lost their homes and gone bankrupt, they were depressed, even suicidal, and they felt unable to embark on a proper relationship," she said.
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