Bromvalerylurea is a sedative-hypnotic agent commonly available in over-the-counter (OTC) medications in Japan and other regions. Although widely used, its prolonged and unsupervised use poses significant risks of dependency and withdrawal. Abrupt discontinuation of bromvalerylurea can lead to severe withdrawal symptoms, including agitation, hallucinations, and seizures. However, standardized management protocols for bromvalerylurea withdrawal have not been established. A 19-year-old male with a suspected history of attention deficit hyperactivity disorder presented with severe agitation, hallucinations, and a generalized tonic-clonic seizure after abruptly discontinuing the sedative containing bromvalerylurea. He had been using an OTC sedative for nine months, progressively increasing his daily intake from three to 24 tablets. Upon admission, his vital signs were stable, but his serum chloride levels exceeded the measurable upper limit due to pseudohyperchloremia caused by bromide, a metabolite of bromvalerylurea. Initial management included oral diazepam 5 mg three times daily, which was tapered over nine days, leading to a resolution of withdrawal symptoms and normalization of chloride levels. The patient was transferred to a psychiatric facility on day 17 for substance use disorder treatment. Drug assays revealed undetectable bromvalerylurea levels by admission, while allylisopropylacetylurea levels declined by day 8. This case underscores the severe withdrawal risks associated with bromvalerylurea, demonstrating the effectiveness of diazepam in stabilizing symptoms during withdrawal. Additionally, pseudohyperchloremia caused by bromide metabolism highlights the importance of recognizing this laboratory finding to avoid unnecessary interventions. With bromvalerylurea products being phased out as OTC medications due to safety concerns, efforts must focus on raising awareness about their risks and preventing misuse through online sales.