Eris is the largest dwarf planet currently known in the solar system. Knowled ge about its physical parameters is necessary to interpret the characteristics of these kinds of bodies. The goal of this work is to study Eris' short-term and long-term variability in order to determine the amplitude of the light-curve, which can be linked to the degree of elongation of the body or to the degree of albedo heterogeneity on the surface of the dwarf planet. In addition, the rotation period can be determined. CCD photometric observations of the trans Neptunian object Eris in R band on 16 nights spanning two years were carried out using the 1.5m telescope at Sierra Nevada Observatory (OSN), the 2.5m Isaac Newton Telescope (INT) telescope at the Roque de los Muchachos Observatory, and the 2.2m Telescope at Calar Alto Observatory. The time-series analysis leads to indications of a short-term variability whose nature is not clear. It could be real or a result of data-reduction artifacts, such as contamination by close, faint-background stars. The most significant periodicities are 14h or its double, but other possibilities cannot be ruled out, like a 32h weaker peak in the periodogram. As for the amplitude of the light-curve, we get a peak-to-peak variability of 0.01+/-0.01mag. The study of the long-term variability indicates that a long rotation period cannot be rejected, but the amplitude would be smaller than 0.06mag. These results are compatible with a nearly spherical body that has a homogeneous surface.