A: There is no absolute value considered to be too low simply because blood pressures vary by person and concomitant disease processes. First I need to know what your blood pressure is usually ( at baseline when you are doing fine). If your blood pressure usually runs 110 to 120 systolic ( the top number), then blood pressures less than 90mmHg are considered low. The common denominator is the presence or absence of symptoms. If you feel light headed, whatever the measured BP, it is too low. The usual causes of low BP are medications. Other causes include anemia, heart attack, heart failure, endocrine problems.
People with hypertension can run blood pressures as high as 200/100 in the resting state. If their blood pressure drops down to 120/80, considered “normal” BP in most people, would be considered catastrophically low in people with hypertension and this would cause the person to be light-headed or even have a stroke! Therefore, what blood pressure is considered “too low” depends upon the individual clinical state of the patient with their underlying disease process. There is, unfortunately, no absolute number for all patients that can be universally be considered too low. That is why a doctor’s prompt input is so important when a person gets dizzy or lightheaded. Get a blood pressure first and then call the doctor. Know what your usual blood pressure is at the baseline state.