Is addiction a disease?

This is disputing definitions [LW · GW]. The proper questions are how the addiction works, and what are its impacts on your life. Those are the important things. Whether than fits under textbook definition of disease, who cares? (Well, the healthcare establishment may care, and you may care indirectly, if it e.g. allows you to apply for disability benefits, etc.)

Or does it effect rationalism?

Does it help you think more clearly? Does it help you achieve your goals?