The main and most important effect of memantine is NMDA antagonism. It has a biological half-life of 60-100 hours, so it is unsuitable for recreational use as a drug.
https://en.wikipedia.org/wiki/Memantine
https://en.wikipedia.org/wiki/NMDA_receptor_antagonist
When treating Alzheimer's disease with memantine alone, without an acetylcholinesterase inhibitor, the treatment will be ineffective, as memantine has a negative side effect of antagonism on acetylcholine receptors. The dose of the drug from the anticholinesterase group must be increased until its side effects begin to appear, such as excessive salivation, cramps in the digestive system, incontinence (the dose must be set just below the manifestation of side effects).
https://en.wikipedia.org/wiki/Memantine#Cholinergic_.28nicotinic_acetylcholine_receptor.29
https://en.wikipedia.org/wiki/Acetylcholinesterase_inhibitor
It is very correct to combine memantine with the drug varenicline https://en.wikipedia.org/wiki/Varenicline
https://en.wikipedia.org/wiki/Varenicline
https://www.adcc.sk/web/humanne-lieky/atc-skupiny/n07ba03/vareniklin.html
Memantine acts against excitotoxicity, so it should be used in the very early stages of Alzheimer's disease (because in the advanced stages of the disease it is already very, very late), but also if not all diagnostic criteria for Alzheimer's disease are met (as a preventive measure). There is no empirical, rational, and logical explanation of how memantine can help cognitive abilities in the advanced stages of the disease based on excitotoxicity. Memantine can only slow down or stop degeneration by excitotoxicity. In the advanced stages, it is already too late to help the patient with memantine.
Doctors in Slovakia don't even know what excitotoxicity is, because it requires a lot of studying https://en.wikipedia.org/wiki/Excitotoxicity
Officially, memantine is indicated only in the moderate to severe stage of the disease. This is because only short-term and small-scale scientific studies have been conducted to directly prove the beneficial effect of memantine in the initial stage of the disease.
Research has apparently shown that memantine improves cognitive abilities in moderate to severe disease. This could be scientifically explained by the fact that a side effect of all NMDA antagonists causes an increase in the volume of nerve cells, because the vacuoles of these cells take up more water. Since an Alzheimer's patient has a shrunken brain, memantine increases the volume of their brain. The volumetrically shrunken brain of an Alzheimer's patient is not capable of doing "Mexican waves", which worsens their cognitive abilities. Memantine increases the volume of their brain, the brain becomes capable of Mexican waves and the patient begins to have better cognitive abilities.
Comparison of an Alzheimer's brain and a healthy brain
https://en.wikipedia.org/wiki/File:Alzheimer%27s_disease_brain_comparison.jpg
Scientific research proving that Mexican brain waves play an important role in cognitive abilities in general and in Alzheimer's disease
http://www.sciencedaily.com/releases/2014/11/141118072634.htm
Increased volume of nerve cells as a side effect of NMDA antagonists
https://en.wikipedia.org/wiki/NMDA_receptor_antagonist
https://en.wikipedia.org/wiki/Olney%27s_lesions
The dose of memantine against excitotoxicity, for preventive indication or for the milder stage of Alzheimer's disease, is sufficient to be small, far less than to manifest side effects of hallucinations.
Also, when choosing the dose size, the doctor must be very careful not to give a higher dose, as side effects of hallucinations will occur. It may not be easy to distinguish what hallucinations are from memantine and what "Alzheimer's psychosis" is.