Addiction in and of itself is not a sin, I believe. True addiction is an illness, like cancer.
Addiction can lesson our ability to recognize a grave matter as sin. It can also lesson our ability to employ our free will not to sin. In these cases, the grave matter we do (theft, battery, etc.) is still a sin, but the addiction can keep it from rising to the level of a mortal sin.
Addiction, though, is not a “free ride”. You can’t say “well, I’m addicted to porn to I don’t have to worry about it because there is nothing I can do”. As was said just above, addiction is a cross we must carry.
If there is a specific addiction or sin that is troubling you or a friend, the better place to discuss this issue would be in confession.