Yes it can become an addiction and yes it is harmful. Not as harmful as some other drugs, but its definitely not good for you. One way to tell if you are addicted or dependent on it, is to scan your body and mind when you run out of it. How are you feeling? If the answer is uneasy, anxious, empty, bored, sad, on edge etc. Than you probably have an addiction problem, its not physically addicting but its psychologically addicting. I was addicted and in denial for many years, quitting was actually really hard.

I'd say its harmful because: it messes with your concentration/attention span. It weakens your immune system because THC works like a steroid, it can also cause men to grow breasts. It messes up your mental health in many ways, interferes with memory, shrinks the part of your brain that regulates emotion (hippocampus). It can lead to depression and make preexisting mental health conditions more severe. It also messes up your central nervous system and can have lasting negative consequences on your blood pressure. Using it causes an increase in anxiety and as splitimage wrote, long term use can lead to paranoia. Even though I quit I'm still pretty paranoid. If you are considering quitting, I would highly recommend you do so while you are ahead. I should've listened to my doctor and quit sooner.