Pornography triggers brain activity in sex addicts similar to the effect drugs have on the brains of drug addicts, researchers said on Friday - but that doesn't necessarily mean porn is addictive.

Although there are no precise figures, experts in the field believe as many as one in 25 adults is affected by compulsive sexual behavior, more commonly known as sex addiction - an obsession with sexual thoughts, feelings or behavior they are unable to control.

Excessive use of pornography is one of the main features of the condition. That can affect personal lives and work, causing distress and feelings of shame, the researchers from Britain's Cambridge University said in a study published in the journal PLOS One.

The study looked at brain activity in 19 male patients affected by sex addiction and compared them with the same number of volunteers. The patients had started watching pornography at earlier ages and in higher proportions than the volunteers.

"The patients in our trial were all people who had substantial difficulties controlling their sexual behavior, and this was having significant consequences for them, affecting their lives and relationships," said Dr Valerie Voon, who led the study at Cambridge's department of psychiatry.

"In many ways, they show similarities in their behavior to patients with drug addictions. We wanted to see if these similarities were reflected in brain activity, too."

The study participants were shown a series of short videos featuring either sexually explicit content or sports. Their brain activity was monitored using functional magnetic resonance imaging, which uses a blood-oxygen-level-dependent signal to measure brain activity.

The researchers found that three regions in particular were more active in the brains of the sex addiction patients compared with the healthy volunteers.

Significantly, these regions - the ventral striatum, dorsal anterior cingulate and amygdala - are also activated in drug addicts when they are shown drug stimuli, the researchers said.

The ventral striatum is involved in processing reward and motivation, while the dorsal anterior cingulate is involved in anticipating rewards and drug craving, they said. The amygdala helps process the significance of events and emotions.

The researchers also asked the participants to rate their levels of sexual desire while watching the videos and say how much they liked them. Drug addicts are thought to be driven to seek their drug because they want it rather than because they enjoy it.

This process is known as incentive motivation, Voon said, and is a compelling theory in addiction disorders.

Patients with sex addiction showed higher levels of desire toward the sexually explicit videos, but did not necessarily like them more.

"Whilst these findings are interesting, it's important to note ... that they could not be used to diagnose the condition," Voon said. "Nor does our research necessarily provide evidence that these individuals are addicted to porn - or that porn is inherently addictive."