QUESTION: What is the lowest blood pressure should go?

ANSWER: The lower limit for blood pressure is perhaps best defined by symptoms rather than numbers.

In the early 1900s, life insurance companies began compiling actuarial data that established a link between increased blood pressure and increased risk of death. This insight led some doctors to adopt a blood pressure philosophy of “the lower, the better.”

Successive guidelines have since decreased the recommended threshold for what constitutes high blood pressure, from 160/95 millimeters of mercury in 1973, to 140/90 in 1997, to 130/80 in 2017.

Still, the question remained: How low should blood pressure go?

A decadelong study that began in 1972 provided a clue.

As expected, the study showed that lowering blood pressure decreased the risk of death. But the risk increased again unexpectedly as the diastolic blood pressure (the second number, which is the pressure as the heart relaxes) fell below 85 millimeters of mercury, a pattern the study authors described as a “J-curve.”

Subsequent studies have been mixed. A 2014 observational study of more than 1 million patients argued that the J-curve does not exist, but a 2018 randomized clinical trial of more than 30,000 patient suggests that it does. Overall, there is little evidence to support lowering systolic or diastolic blood pressure below the levels of current guidelines.

Doctors do agree, however, that symptoms are an unequivocal sign of excessively low blood pressure.

Lightheadedness, particularly lightheadedness that occurs upon standing, is the cardinal symptom of low blood pressure. At its extreme, lightheadedness may result in fainting. Fainting may be a sign that blood pressure is dangerously low and requires prompt medical attention.

Other possible symptoms of low blood pressure include nausea, fatigue and blurry vision.

Symptoms of low blood pressure can sometimes be a result of too much blood pressure medication or too many blood pressure drugs. If you have symptoms, your doctor may decrease your dose, discontinue one of your medications or switch to a different medication.

Your doctor can also evaluate whether you have heart problems or other conditions that may be making your blood pressure low.

Because many questions remain unanswered about optimal blood pressure management, and because the consequences of excessively low blood pressure can be serious, it is essential to stay within guideline recommendations. Doing so is the best health advice for the 75 million people — or roughly 1 in 3 adults in the United States — who have high blood pressure.