The first study to investigate brain activity in "sex addicts" has uncovered remarkable similarities with people addicted to drugs.
There is some controversy about whether people can become addicted to sexual behaviours, including watching porn.
University of Cambridge researchers performed brain scans on 19 men watching pornographic videos.
They showed the same reward centres of the brain were activated as when addicts see their drug of choice.
Two of the men in the study had lost jobs through watching pornographic material at work.
Four of them say porn was a gateway drug to escort agencies and fetishes.
They were all obsessed with sexual thoughts and behaviour, but it is uncertain that they are "addicts" in the same way as a smoker is addicted to nicotine.
Some researchers argue their traits more closely resemble an impulse control or obsessive compulsive disorder.
A team of researchers used functional magnetic resonance imaging to observe the changes in brain activity caused by watching pornographic videos.
They compared the brains of people with compulsive sexual behaviour and healthy people.
The results, published in the journal PLoS One, showed higher levels of brain activity in the three parts of the the brain: the ventral striatum, dorsal anterior cingulate and the amygdala.
These are all areas that become excited in addicts when they see their drug of choice.
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.
"We don't know if some of these effects are predispositions, meaning that if you have greater activity in these areas are you more likely to develop these behaviours or if it is an effect of the pornography itself - it's very difficult to tell."
She added that the earlier people were exposed to drugs the more likely they were to develop into addicts.
However, Dr Voon cautioned there was insufficient evidence to advise on the impact of teenagers watching porn on the internet.
'Lies and deceit'
Paula Hall, the chair of the Association for the Treatment of Sex Addiction and Compulsivity, said the internet was now providing non-stop sexual novelty.
She told the BBC News website: "What I'm seeing is increasing numbers of young men who cannot maintain an erection because they've wrecked their appetite with pornography, their arousal threshold goes up so a mere mortal doesn't do it anymore.
"The damage to a couple's relationship can be huge, the lies and deceit, not having sex with partner as they appear to present with a low sex drive as they're on the internet all the time.
"The might stop engaging in family activities as it's a chance to have the house to yourself, porn is often where it starts and it's a bit of a gateway drug to sex workers."
However, she said it was still controversial to say such people were addicts and the field was poorly researched.
Dr John Williams, the head of neuroscience and mental health at the Wellcome Trust charity, says: "Compulsive behaviours, including watching porn to excess, over-eating and gambling, are increasingly common.
"This study takes us a step further to finding out why we carry on repeating behaviours that we know are potentially damaging to us.
"Whether we are tackling sex addiction, substance abuse or eating disorders, knowing how best, and when, to intervene in order to break the cycle is an important goal of this research."