Blood pressure lower than 120/80 mm Hg is considered “normal,” and the term “low blood pressure” is relative. Low blood pressure is generally considered dangerous when it drops suddenly or is accompanied by symptoms, such as dizziness or fainting. Severely low blood pressure can indicate serious heart, endocrine or neurological disorders and can deprive the brain and other vital organs of oxygen and nutrients, leading to shock, which can be a life-threatening condition.

Symptoms of low blood pressure to watch for include:

*Dizziness or lightheadedness
*Fainting (called syncope)
*Lack of concentration
*Blurred vision
*Nausea
*Cold, clammy, pale skin
*Rapid, shallow breathing
*Fatigue
*Depression
*Unusual thirst
There is no specific number at which blood pressure is considered too low. Most doctors consider chronically low blood pressure dangerous only if it causes noticeable signs and symptoms. However, a sudden fall in blood pressure can be dangerous — even a change of just 20 mm Hg can cause dizziness or fainting. Some rapid falls in blood pressure indicate a deeper underlying problem such as uncontrolled bleeding, severe infections or allergic reaction.