Abstract: The invention relates to a compound (RS)-2-(2-oxo-4-phenylpyrrolidin-1-yl)acetamide having modulatory activity with a commensurate effect, to a pharmaceutical substance (RS)-2-(2-oxo-4-phenylpyrrolidin-1-yl)acetamide containing: 2-(2-oxo-4-phenylpyrrolidin-1-yl)acetamide—no less than 99.0% and no more than 100.5% on dry basis, individual related impurities individually or in total—no more than 0.2%; residual amounts of organic solvents individually or in total—no more than 3000 ppm. The invention also relates to a method for producing the pharmaceutical substance differing in that the obtained raw materials are subjected to purification, crystallization and stabilization of the compound by processing it with demineralized (distilled) water, and the isothermal crystallization from propanol followed by drying. The invention also relates to compositions for internal and external application. The invention provides higher efficacy, safety, stability and width of therapeutic application.