High blood pressure is a common condition where the force of blood against artery walls is too high and can cause health problems over time. Most people have no symptoms, even at dangerously high levels, though some may experience headaches, dizziness, or nosebleeds. A doctor diagnoses high blood pressure through multiple blood pressure tests over time, with a reading of 140/90 mmHg or higher considered high. Blood pressure is tested through a painless test in a medical office after preparing by not drinking coffee or smoking, using the bathroom, and sitting for 5 minutes beforehand.