A 52-week, double-blind, randomised, multi-centre, phase III, parallel-group study in patients 12 years and older with asthma, evaluating the efficacy and safety of Symbicort (budesonide/formoterol) Turbuhaler 160/4.5 μg 'as needed' compared with Pulmicort (budesonide) Turbuhaler 200 μg twice daily plus terbutaline Turbuhaler 0.4 mg 'as needed'