What are the effects of stopping concerta cold turkey?

Depends: In theory, you can stop Concerta any time, and many people do, e.g., skipping it on weekends. The symptoms you are treating will come back immediately, but that should be about it. However, if you are taking extremely high doses, say over 90 mg a day, you might find yourself have irritability and perhaps sleep problems for a few days. This is true of any stimulant.

Concerta: Lack of focusing, distractibility, some increase appetite.