Following an application from SmithKline Beecham Limited, submitted for authorisation of a health claim pursuant to Article 13(5) of Regulation (EC) No 1924/2006 via the Competent Authority of United Kingdom, the EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA) was asked to deliver an opinion on the scientific substantiation of a health claim related to caffeine and increased alertness. The food constituent, caffeine, which is the subject of the health claim, is sufficiently characterised. Increased alertness might be a beneficial physiological effect. A claim on caffeine and increased alertness, in the general adult population, for products containing at least 75 mg of caffeine per serving, has already been assessed by the Panel with a favourable outcome. In the present application, the applicant proposed that, in order to bear the claim, a product should contain at least 40 mg of caffeine per serving. In weighing the evidence, the Panel took into account that most studies which measured reaction time in various cognitive tasks found no effect of caffeine at doses < 75 mg. In the particular dose range between 40 and < 75 mg, no effect of caffeine was found on the majority of outcome measures of reaction time. The Panel notes that the majority of studies with caffeine doses of 75 mg or higher showed a significant reduction in measures of reaction time. On the basis of the evidence provided, the Panel reiterates its previous conclusion that, in order to bear the claim, a product should contain at least 75 mg caffeine per serving. The Panel concludes that a cause and effect relationship has not been established between the consumption of caffeine and increased alertness under the conditions of use proposed by the applicant.