M stars are very abundant in our Galaxy, and very likely harbour the majority of planetary systems. A particularity of M stars is that they are very active. They experience stellar variability such as flares for instance. These violent and unpredictable outbursts originate from the photosphere and are caused by magnetic processus. During such an event, the energy emitted by the star can vary by several orders of magnitude for the whole wavelength range. It results in an enhancement of the H alpha emission and of the continuum. Some studies on the effect of flares on exoplanets have already been conducted (Segura et al. 2010, Tofflemire et al. 2012). Here we are interested in the effect of a flare on the atmospheric composition of a super-Earth orbiting around an M star. Using the stellar flux of AD Leo recorded during a flare event (Hawley & Pettersen 1991, Segura et al. 2010) and the chemical model of Venot et al. 2012, we have studied the impact on the planetary atmosphere. We have also computed the synthetic spectra assuming that such an event occurs during a transit to see in which extent a flare can influence observational data. We will present these results.