Readings at or above 180/120 mmHg are considered a hypertensive crisis and need prompt attention. If such a reading comes with symptoms, doctors call it a hypertensive emergency: warning signs include chest pain, breathlessness, severe headache, blurred vision, confusion, weakness on one side, or difficulty speaking, which suggest the high pressure is damaging organs.
If you get a reading this high with any of those symptoms, call your local emergency number right away. If the reading is that high but you feel completely well, rest quietly for a few minutes and measure again; if it stays very high, contact a doctor urgently the same day.
Do not try to bring the pressure down quickly yourself with extra doses of medication, as rapid drops can be harmful. Lowering very high blood pressure safely needs medical supervision.