Is drug addiction considered a mental illness?

Technically, I don't know, but I do know that mental health care professionals are a big part of the team that help treat drug addiction. Often people addicted to drugs have other things going on in their lives (like depression or an anxiety disorder) that needs to be treated if they're going to get clean and stay that way. Some people who use drugs are actually medicating a mental illness they're not aware of.

My Answer

It is. The brain wants more and more of the chemical, say nicotine in cigarettes. As soon as you inject nicotine, a feeling of relaxation is what you get. To be in detail, Synapses are formed between two neurons by a certain chemical. Nicotine replaces the chemical and helps in the formation of synapse. Thus it is related to mental illness. Same applies to any drug I guess.