Review Question - QID 214926

A 45-year-old woman presents to the primary care clinic with 1 week of debilitating facial pain. The pain is episodic, stabbing in quality, and shoots across her right cheek. This morning, her pain was so severe while she was eating her breakfast that she had to stop. She denies prior facial trauma or headache. The patient has a history of diabetes for which she takes metformin. Her temperature is 98.8°F (37.1°C), blood pressure is 134/86 mmHg, pulse is 92/min, and respirations are 16/min. On physical exam, there is no palpable clicking of the jaw or crepitus during mouth opening and closing. There is no facial rash. Which of the following side effects is most associated with the first-line treatment for her condition?