Taking a single Olmezest 20 mg (olmesartan) tablet without having high blood pressure can temporarily lower your blood pressure. You may experience dizziness, lightheadedness, or a mild headache, particularly when standing up quickly. To stay safe, sit or lie down immediately if you feel dizzy, and drink plenty of water to help stabilize your blood pressure. Avoid sudden movements. This effect is temporary and should wear off within 24 hours as the drug leaves your system. However, if you experience severe dizziness, fainting, or chest discomfort, please seek immediate medical attention.