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The American author Mark Twain is often, possibly apocryphally, quoted as saying that quitting smoking was easy - he'd done it 100 times.

As a society we accept the existence of addictions to substances, such as nicotine, alcohol and other drugs - and the harm they can cause. But when it comes to sex, some experts still disagree over whether addiction is real or a myth.

Sex addiction is currently not a clinical diagnosis, which means we don't have official figures on how many people have sought help for related concerns through the NHS.

A self-help website for people who feel they are struggling with sex or porn addiction surveyed 21,000 people in the UK who have visited the site for help since 2013.

Of these, 91% were male and only 10% had sought help from a GP.

Sex addiction was considered for inclusion in the latest 2013 edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM), a key diagnostic tool in both the US and the UK, but was rejected because of lack of evidence.

But "compulsive sexual behaviour" is now being proposed as an entry in the International Classification of Disease (ICD) manual produced by the World Health Organization.

Gambling was previously considered in the category of compulsive behaviours, but was given formal diagnostic status as an addiction in 2013, along with binge-eating disorder, after new evidence emerged.

Therapists believe sex addiction could follow a similar route.

Image copyright Science Photo Library Image caption Researchers found watching porn activated the same reward pathways in the brain as those activated by taking drugs.

A study published in 2014 suggested brain activity in "sex addicts" watching porn is similar to that of drug addicts when shown their drug of choice.

At the time, its lead researcher Dr Valerie Voon, from the University of Cambridge, told the BBC: "This is the first study to look at people suffering from these disorders and look at their brain activity, but I don't think we understand enough right now to say it is clearly an addiction."

Whether you believe someone can be addicted to sex will depend largely on what you think makes something an addiction - and there's no one officially accepted definition.

If it's purely something on which someone becomes physiologically reliant - that withdrawing from would cause physical harm - then sex "can't be an addiction," according to Dr Frederick Toates, an emeritus professor at the Open University.

But he believes a broader definition is more useful.

'Reward-seeking'

Dr Toates says there are two key features that mark out an addiction: the seeking of reward or pleasure, and the existence of conflict around this behaviour. The seeking of reward is what many experts believe distinguishes addiction from obsessive compulsive behaviour, although there are striking similarities.

People with an addiction are seeking a short-term gain, even if this may be outweighed by the long-term loss. In contrast, people with obsessive compulsive disorder engage in behaviours from which they derive no pleasure, he says.

But we all seek out pleasure, so what separates regular reward-seeking behaviour from addiction?

Psychologist Dr Harriet Garrod thinks a behaviour becomes an addiction when it reaches a level of intensity such that it causes harm to the individual and those around them.

Addictions to food and gambling have been recognised as diagnosable conditions while sex addiction hasn't, because they have been in the public consciousness for longer, she says.

This means more people have come forward seeking clinical help, providing more evidence to support their existence as a condition, according to Dr Garrod.

Media playback is unsupported on your device Media caption Rebecca Barker said sex addiction took over her life and ruined her relationship

Dr Abigael San is a clinical psychologist who believes that sexual behaviours could be addictive, but for people struggling with feeling out of control, the sex itself is secondary to the underlying problems - whether that's depression, anxiety or trauma - that leads them to use it as a coping mechanism.

"Different activities and substances activate reward pathways in different ways but they still activate those reward pathways," she says.

"There's no reason to believe sex doesn't work in the same way - it's just we don't have enough evidence for it yet."

But she's not convinced labelling it as an addiction would necessarily help people, especially those using sex to cope with other issues - and it could lead to over-diagnosis.

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Sex addiction 'myth'?

Not everyone agrees that sex addiction is a real condition, however.

David Ley, a sex therapist who wrote The Myth of Sex Addiction, says behaviours commonly labelled as sex addiction are in fact the symptoms of untreated mood and anxiety disorders and that evidence for its treatment was lacking.

"Equating sex or masturbation to alcohol and drugs is ludicrous. People addicted to alcohol can die from withdrawal," he said.

He adds that "the concepts of sex addiction are based on moral values of what healthy sex is".

"You're a sex addict if you have more sex, or different sex, than the therapist diagnosing you."

In a research paper on the inclusion of compulsive sexual behaviour in the next edition of the International Classification of Disease, a group of researchers, including Dr Voon, is keen to avoid this trap.

They say the diagnosis should not be used "to describe high levels of sexual interest and behaviour" or based on "psychological distress related to moral judgments or disapproval about sexual impulses".

But for them, and others who want the condition to be formally recognised, having a clinical label is about ensuring people in distress can receive help - whether the addictive behaviour is the problem in itself or the symptom of another deeper issue.