A 46-year-old male has been taking venlafaxine XR 225 mg daily for 2 years for major depressive disorder with sustained remission. He ran out of medication 3 days ago while traveling and could not get a refill. He presents to an urgent care clinic reporting intense dizziness, electric shock-like sensations radiating through his head and limbs ('brain zaps'), nausea, irritability, vivid nightmares, and a sensation of disequilibrium when moving his eyes. Vital signs are normal and he denies suicidal ideation. Which of the following best explains this presentation?