Cystic fibrosis is a life-threatening genetic disease mainly characterised by accumulation of viscous secretions in the airways. In the absence of a better alternative, inhaled dornase alfa is used to liquefy bronchial secretions and thereby facilitate their drainage. Mannitol, in the form of capsules of powder for inhalation, is authorised in the European Union for use as a mucolytic in adults with cystic fibrosis. Two double-blind randomised trials have compared two doses of inhaled mannitol (400 mg or 50 mg, twice a day) in a total of 642 patients (57% adults) with cystic fibrosis. After 26 weeks of treatment, there was no difference between the groups in terms of clinical criteria such as the frequency of pulmonary exacerbations, quality of life, hospitalisation, or rescue antibiotic use. Inhaled mannitol increases the risk of bronchospasm and can also cause coughing and haemoptysis. A pretreatment test, used to exclude patients with bronchial hyperresponsiveness to mannitol, can also have noteworthy adverse effects. Treatment is inconvenient, requiring inhalation of the contents of 10 mannitol capsules morning and evening; the capsules have to be placed one by one in the inhalation device, and the device must be replaced every week. In practice, patients with cystic fibrosis would be well advised to avoid inhaled mannitol.