There isn't much question that porn has the potential to be addictive, because of the way it affects biochemical processes in the brain that involve sexual arousal. The arousal affects the same endorphin and serotonin systems that many recreational drugs do, albeit in slightly different ways.

However, for most people, the stimulus and the affect on the receptors affected is mild in comparison with recreational drugs (opioids, etc) and so for most people the addiction risk is pretty minimal.

For people that have addictive personalities or are genetically predisposed to sensitivity with the endorphin and serotonin systems, porn may be extremely addictive, but then so would a number of other things (video games, for example) that tend to stimulate the same biochemical processes.