Methyldopa sesquihydrate (C10H13NO4·1.5H2O) is a substance used in the treatment of hypertension being one of the most widely used drugs in the world. In this work we have studied the stability of this drug as a function of temperature using four different techniques: X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry and Raman spectroscopy. From the combination of these techniques we were able to infer about a set of phase transitions during the loss of water from the crystal around the temperatures of 388 K (changes only in the lattice modes) and between temperatures 393 and 423 K, when all water leaves the crystal and changes in some lattice modes and in some internal modes occur.