At Visit 3 the therapist can increase the dose to the double of the initial dose according to clinical response and tolerability. At Visit 4 the dose can be increased to the double of the last dose or reduced to the initial dose. At Visit 5 the most effective and best tolerated dose will be given for the 6 week fixed dose phase of the trial. Therefore the maximum daily dose is 20 mg (patients with body weight ≤ 50 kg) or 40 mg (patients with body weight > 50 kg). The total dose will be split and the half-dose will be given twice a day (morning and evening).