Been on the same dose of concerta for 18mos. the last several weeks it doesn't feel like it's working. what gives?

Speak to your doctor: Many things can change the effectiveness of medications, including Concerta. If you are on other medications, eating differently, under new stresses, sleeping differently and possible other changes, the efficacy of your medication may change. Speak to your prescribing doctor to see what your options are.

Concerta Tolerance: Unfortunately, this is not uncommon with the active ingredient of Concerta which is methylphenidate, a.k.a. Ritalin. I don't know what dosage you are taking nor whether it has ever been changed. Talk with your doctor about taking some "drug holidays". Perhaps on the weekends you can go without taking the Concerta. This sometimes helps to "trick" the body into not developing a tolerance to a drug