Abstract: The invention relates to an area of the chemical-pharmaceutical industry and medicine, particularly to an agent for treatment of dementias, including Alzheimer's disease (AD), as well as to a method of treatment and pharmaceutical composition, which is effective for both treatment of dementias, including AD or dementias of Alzheimer type, and during manifestations of the first symptoms of memory disorder. The given invention consists of the development of a new therapeutic substance with minimal side effects that currently is a topical issue for treatment of the corresponding diseases. Attached is the use of polyprenols of formula (1) where n=8-20 as an active ingredient for the production of the therapeutic agent for the treatment of patients with dementia syndrome, including those who suffer from Alzheimer's disease. It is proposed to use a therapeutic agent for the treatment of patients with dementia syndrome, including those suffering from Alzheimer's disease, being polyprenols of formula (1).