Pharmaceutical compositions with anti-obesity activity that act peripherally are provided, which comprise a premixture made up of pure orlistat as the active ingredient and other components that afford the premixture stability and suitable physical properties for simply preparing compositions for oral use with convenient dosage flexibility; and optionally necessary thickening, flavouring and colouring agents. A method for preparing said compositions is also provided. The orlistat content in the premixture is less than 20% of the total weight of the mass, preferably between 12 and 17%.The pharmaceutical composition can be formulated indistinctively from the premixture both as grooved tablets and powder for suspension.