I've been taking 10 mg ritalin (methylphenidate) twice a day for three weeks. i no longer feel the effects. have i developed a tolerance? should i ask for ritalin (methylphenidate) xr?

Tolerance OK term: I am glad you use the term tolerance which is a decrease of the effect of the medication over time; the person becomes accustomed to it. It is different from addiction (craving, withdrawal symptoms, chronic compulsive use; and different from physical dependence). A dose of 10 mg of Ritalin (methylphenidate) twice a day is a good therapeutic dose. I do not know why you take it but ask your health provider (nuvigil?).