A formulation of a therapeutic or diagnostic agent in the form of a nano-emulsion, comprising a continuous aqueous phase and at least one dispersed oily phase, wherein: - the oily phase comprises: - at least one amphiphilic lipid, - at least one solubilizing lipid, - at least one compound of the following formula: R − L n − A wherein: - n represents an integer from 1 to 5, notably 1 to 2, preferably 1, - L represents a simple bond or a divalent group selected from -O-, -COO-, -OOC-, -CO-NR'-, -NR'-CO-, -S-, -NR'-CO-NR"-, -O-CO-O-, wherein R' and R" represent independently H or a linear or branched alkyl with 1 to 20 carbon atoms and - R represents a linear or branched alkyl comprising at least 11 carbon atoms or a linear or branched alkenyl comprising at least 11 carbon atoms, and - A represents a steriodic or sterolic group, and - the aqueous phase includes at least one co-surfactant comprising at least one chain consisting of alkylene oxide units the proportion of the compound of formula being from 0.05 to 3% by weight based on the weight of the dispersed oily phase, the weight of the dispersed oily phase including the weight of the co-surfactant. - the oily phase comprises: - at least one amphiphilic lipid, - at least one solubilizing lipid, - at least one compound of the following formula: R − L n − A wherein: - n represents an integer from 1 to 5, notably 1 to 2, preferably 1, - L represents a simple bond or a divalent group selected from -O-, -COO-, -OOC-, -CO-NR'-, -NR'-CO-, -S-, -NR'-CO-NR"-, -O-CO-O-, wherein R' and R" represent independently H or a linear or branched alkyl with 1 to 20 carbon atoms and - R represents a linear or branched alkyl comprising at least 11 carbon atoms or a linear or branched alkenyl comprising at least 11 carbon atoms, and - A represents a steriodic or sterolic group, and - at least one amphiphilic lipid, - at least one solubilizing lipid, - at least one compound of the following formula: R − L n − A wherein: - n represents an integer from 1 to 5, notably 1 to 2, preferably 1, - L represents a simple bond or a divalent group selected from -O-, -COO-, -OOC-, -CO-NR'-, -NR'-CO-, -S-, -NR'-CO-NR"-, -O-CO-O-, wherein R' and R" represent independently H or a linear or branched alkyl with 1 to 20 carbon atoms and - R represents a linear or branched alkyl comprising at least 11 carbon atoms or a linear or branched alkenyl comprising at least 11 carbon atoms, and - A represents a steriodic or sterolic group, and - n represents an integer from 1 to 5, notably 1 to 2, preferably 1, - L represents a simple bond or a divalent group selected from -O-, -COO-, -OOC-, -CO-NR'-, -NR'-CO-, -S-, -NR'-CO-NR"-, -O-CO-O-, wherein R' and R" represent independently H or a linear or branched alkyl with 1 to 20 carbon atoms and - R represents a linear or branched alkyl comprising at least 11 carbon atoms or a linear or branched alkenyl comprising at least 11 carbon atoms, and - A represents a steriodic or sterolic group, and - the aqueous phase includes at least one co-surfactant comprising at least one chain consisting of alkylene oxide units the proportion of the compound of formula being from 0.05 to 3% by weight based on the weight of the dispersed oily phase, the weight of the dispersed oily phase including the weight of the co-surfactant.