The European Commission granted a marketing authorization valid throughout the European Union for Nucala on 2 December 2015. Nucala is an asthma medicine used to treat adults with a particular type of asthma called eosinophilic asthma. It is used with other medicines in patients whose asthma is severe and not well controlled with previous treatments. In eosinophilic asthma, symptoms are associated with having too many of a type of white blood cell called eosinophils in the blood and in phlegm in the lungs. The active substance in Nucala, mepolizumab, is a type of protein called a monoclonal antibody, which attaches to a specific substance in the body. Mepolizumab attaches to a substance called interleukin-5 which encourages the production and survival of eosinophils. By attaching to interleukin-5, mepolizumab blocks its action and thereby reduces the numbers of eosinophils. This helps to reduce inflammation, resulting in a reduction in asthma attacks and improvement of symptoms. The benefits of Nucala in severe eosinophilic asthma that is not well controlled by previous treatment have been shown in three main studies, in which it was compared with a placebo (dummy) injection. The first study involved 616 adults and adolescents given Nucala every 4 weeks for a year, in addition to their regular asthma medicines. The second study involved 576 adults and adolescents given Nucala every 4 weeks for 28 weeks. The main measure of effectiveness in these studies was the number of severe attacks (exacerbations) of asthma that occurred during treatment, which was reduced by about half in patients given Nucala. The third study involved 135 patients with eosinophilic asthma severe enough to need regular treatment by mouth with corticosteroids (potent anti-inflammatory medicines such as prednisone and prednisolone), and the main measure of effectiveness was how much the corticosteroid dose could be reduced using Nucala for 24 weeks compared with placebo. Over half (37 of 69) of the patients given Nucala were able to reduce their daily corticosteroid dose by more than 50% to a dose of 5 mg or less, and 10 of them were able to stop corticosteroids altogether, compared with about a third of those given placebo (22 of 66, of whom 5 were able to stop corticosteroids). The most common side effect with Nucala is headache; reactions at the site of injection and back pain are also common, affecting up to 1 patient in 10.