Well, the answer is that there is no definitive answer to that, yet. As with many behavioral addictions, including sex addiction, that would be a broader category, pornography addiction is just now starting to be researched and understood. In order to give a clear view of how polarizing this topic is, I will bring two articles to this discussion. The first one, by Hilton and Watts (2011), defends that there is a pornography addiction. It starts of by postulating how other addictions affect the brain:
All addictions create, in addition to chemical changes in the brain, anatomical and pathological changes which result in various manifestations of cerebral dysfunction collectively labeled hypofrontal syndromes. In these syndromes, the underlying defect, reduced to its simplest description, is damage to the “braking system” of the brain.
Then they follow the argumentation by saying that other behaviors, such as compulsive eating, can also trigger those changes.
They quote a famous researcher:
A decade ago Dr. Howard Shaffer at Harvard wrote, “I had great difficulty with my own colleagues when I suggested that a lot of addiction is the result of experience … repetitive, high-emotion, high-frequency experience. But it’s become clear that neuroadaptation—that is, changes in neural circuitry that help perpetuate the behavior—occurs even in the absence of drug-taking.”
After a few more examples and quotations from other studies, they conclude:
Certainly our role as healers suggests we can do more to investigate and treat human pathology related to this new entity of process or natural addiction, particularly given the growing weight of evidence supporting the neural basis of all addictive processes. Just as we consider food addiction as having a biologic basis, with no moral overlay or value-laden terminology, it is time we looked at pornography and other forms of sexual addiction with the same objective eye. Currently, social pressures relegate the management of pornography primarily to proceedings in civil or in criminal judicial venues.
This article had a response letter, by Reid et al (2011), which was way more conservative. They state that the article by Hilton and Watts over-generalized the findings of small studies and ignored a lot of gaps of knowledge to postulate pornography addiction as real. Generally, they refute the main points of the first article. This sums up their conclusions:
Thus, although for some it is common to speak of pornography addiction or other sexual addiction, the lack of convergence of findings has led a growing number to take a more modest and careful position, wherein the connections to other classes of excessive behavior patterns are still under study. Further, delineation of what constitutes an addiction has no agreed-upon standard.
This is also of great importance, since those two aspects are present in all drug addictions:
Research on tolerance or withdrawal, genetic associations, and neuroimaging in hypersexual patients with pornography problems are non-existent at this time.
They conclude that more research is needed, and that there might be more than one explanation to excessive pornography use (that is, some may be addicted to it, some may be using it a lot for another reason).
In a subsequent study, Reid et al (2012) tested the proposed criteria for Hypersexual Disorder for the (then) upcoming DSM-5. Hypersexual Disorder was a category which would include excessive pornography use among its possible presentations. They reported:
The HD criteria proposed by the DSM-5 Work Group on Sexual and Gender Identity Disorders appear to demonstrate high reliability and validity when applied to patients in a clinical setting among a group of raters with modest training on assessing HD.
Despite all that, The American Psychiatric Association opted to not include Hypersexual Behavior on DSM-V because it still lacked evidence to be considered a real diagnosis. Therefore, it is not an official diagnosis as of now, but there is sure to be more research on this and it might change in the future.