The invention relates to the chemical and pharmaceutical industry and to medicine, in particular, to medicinal agents based on a mixture of natural oligomers of isoprenol (Ropren), which inhibits monoamine oxidase (MAO) activity; and to a method for treatment of patients suffering from diseases associated with the excessive activity of monoamine oxidase.The given invention consists of the development of a new therapeutic substance with minimal side effects that is currently a topical issue for treatment of the corresponding diseases.The proposed: Therapeutic substance consisted of polyprenols of formula (1) for treatment of patients with diseases associated with excessive activity of monoamine oxidase. with content of polyprenols of formula (1) in the given therapeutic substance from 0.1 to 80%. The treatment method of diseases associated with excessive activity of monoamine oxidase, which consists of administration to the patients of a single or multiple doses of polyprenols of formula (1) in the amount of 1 to 150 mg per day, in the form of an individual agent or as a pharmaceutical composition including excipients.