Known side effects of Ritalin include:
· an allergic reaction (difficulty breathing; closing of the throat; swelling of the lips, tongue, or face; or hives);
· an irregular or fast heartbeat;
· chest pains or very high blood pressure (blurred vision, severe headache, flushing);
· unusual behavior or confusion; or
· liver damage (yellowing of the skin or eyes, nausea, abdominal pain or discomfort, unusual bleeding or bruising, severe fatigue).

• Other, less serious side effects may be more likely to occur. Continue to take methylphenidate and talk to your doctor if you experience
· insomnia (difficulty sleeping);
· nervousness;
· drowsiness;
· dizziness;
· headache;
· blurred vision;
· tics (repetitive movements);
· abdominal pain, nausea, or vomiting;
· decreased appetite or weight loss; or
· slower weight gain and / or growth.

Source: http://my.webmd.com/content/article/4046.1616