The action stage is the focus for many people attempting to overcome addiction. This is the stage at which real change -- change of behavior -- starts happening. The action stage is typically stressful, but with good preparation, it can also be an exciting time that gives way to new options.
For many people, the action stage starts in a detox or treatment center, where there are trained professionals on site to support you through the early phases of discontinuing an addiction. For others, particularly those whose goals are around moderating or controlling behavior (rather than quitting completely), it can be similar to your usual life, but with greater restraint and perhaps a greater need for support and other ways of coping with stress.
Depending on the goals you set in the contemplation stage, and the plans you made in the preparation stage, the action stage can occur in small, gradual steps, or it can be a complete life change. It may feel strange and even empty to be living life without the drama of your addiction. It takes time to get used to life without an addiction, even if your support and alternative ways of coping are good.
Identifying and developing effective ways of coping with stress are crucial during the action stage. This will allow you to effectively move on to the maintenance stage, without experiencing the relapse stage.
Sources:
Hartney, E., Orford, J., Dalton, S., Ferrins-Brown, M., Kerr, C. & Maslin, J. "Untreated Heavy Drinkers: a Qualitative and Quantitative Study of Dependence and Readiness to Change." Addiction Research & Theory, 11:317-337. 2003.
Prochaska, J., Velicer, W., Redding, C., Rossi, J., Goldstein, M., DePue, J., Greene, G., Rossi, S., Sun, X., Fava, J., Laforge, R., Rakowski, W. & Plummer, B. "Stage-based expert systems to guide a population of primary care patients to quit smoking, eat healthier, prevent skin cancer, and receive regular mammograms." Preventive Medicine, 41:406-416. 2005.
Segana, C., Borlanda, R. & Greenwood, K. "Can transtheoretical model measures predict relapse from the action stage of change among ex-smokers who quit after calling a quitline?" Addict Behav. 31:414-428. 2006.
Velicer, W. F., Hughes, S. L., Fava, J. L., Prochaska, J. O. & DiClemente, C. C. "An empirical typology of subjects within stage of change." Addict Behav. 20:299-320. 1995.

