Background and Objectives: Phenibut (4-amino-3-phenyl-butyric acid), acting as a GABA-B receptor agonist, has a beneficial effect on anxiety. Although its medical use is not approved in western countries, it can be easily obtained worldwide via the Internet, so it spread as a substance of abuse. In recent years, some case reports have, therefore, highlighted episodes of acute toxicity or withdrawal, but it is still a largely unknown phenomenon. Methods: In this case report, a 50-year-old woman was admitted to the emergency room with psychomotor agitation, psychotic symptoms, and insomnia, and was non-responsive to treatment. She was hospitalized at the psychiatry ward for 25 days and gave her consent for the publication of the present case report. Results: The suspicion of phenibut withdrawal allowed to establish the appropriate management, leading to the restitutio ad integrum of the psychopathological case. Conclusions: In the face of an incoercible psychomotor agitation case, the knowledge of the so-called novel psychoactive substances allows for more appropriate clinical management of intoxication and withdrawal syndromes. This is a scientifically significant report as it provides therapeutic and outcome data concerning a syndrome that is still quite unfamiliar.