It has been proven in many clinical and epidemiological studies that folate deficiency leads to neuropsychiatric diseases, e.g. depression, psychoses, epilepsy, or various myeloneuropathies. Folate deficiency also reduces the synthesis of S-adenosylmethionine (SAN), which plays an important role in the synthesis and catabolism of ketocholamines in the brain. Disruption of these processes can lead to disruption of neurotransmission, the symptoms of which may be cognitive decline and mood changes. SAN deficiencies were found both in patients with depression and in Alzheimer’s disease. There are more and more indications that SAN plays a very important role in the process of myelination. Inadequate methylation resulting from SAN deficiency leads to myelin degeneration. It has been shown that administering folic acid to patients with demyelinating disease reverses the symptoms of demyelination.