Reversible cerebral vasoconstriction syndrome (RCVS) is characterized by recurrent thunderclap headaches and reversible multifocal vasoconstriction of cerebral arteries. It may occur spontaneously or be triggered by various factors, including sexual activity, antidepressants, vasoactive drugs, immunosuppressants, fibromuscular dysplasia, Guillain–Barré syndrome, intracranial hypotension, and coronavirus disease 2019 (COVID-19) vaccination. Women appear more susceptible, and RCVS has been reported as a rare transfusion reaction after correction of chronic, severe anemia. A 42-year-old woman with chronic migraine-like headaches and analgesic overuse underwent elective gallbladder surgery. One week prior, she had taken fexofenadine for allergies. During surgery, she developed anaphylactic shock, followed by torpor and right-sided hemiparesis. Magnetic resonance imaging (MRI) revealed multiple ischemic lesions in different arterial territories, with MR angiography showing multifocal vasoconstriction, consistent with RCVS. The patient worsened with intracranial hypertension due to cerebellar edema and brainstem compression, necessitating an emergency posterior fossa decompressive craniectomy. She gradually recovered, regaining mobility and speech over 6 months. However, she continued to experience right homonymous hemianopia and behavioral disturbances, including irritability and reduced spontaneity. This case highlights key aspects of RCVS, including potential triggers, severe neurological complications, and recovery variability. While RCVS is often self-limiting, severe cases can lead to ischemia and life-threatening intracranial hypertension. Diagnosis is based on clinical and radiological findings, and differentiation from conditions such as subarachnoid hemorrhage, eclampsia, and posterior reversible encephalopathy syndrome is crucial. RCVS remains underrecognized but can have severe complications. Early recognition and appropriate management are essential to improving outcomes.