A normal blood pressure for an adult is defined as 90 to 119 systolic over 60 to 79 diastolic. The range between 120 to 139 systolic and 80 to 89 diastolic is called pre-hypertension, and readings above that indicate hypertension, or high blood pressure.
Everyone's blood pressure varies somewhat through the day and night. Too high or too low are both problematic. Your body is generally quite good at making adjustments and keeping your blood pressure in range.
Low pressure is defined as less that 90/60. However, the symptoms of low blood pressure are more important than the numbers. Low blood pressure, known as hypotension, can cause lightheaded spells or fainting (syncope). Fainting can lead to serious injury, broken bones, head trauma, etc. Low blood pressure needs to treated if it is causing symptoms.
Your health care team needs to look for the cause and try to correct it. The most common problem is "orthostatic hypotension", meaning your blood pressure goes down when you stand up. This can occur if you are dehydrated (either because you are losing fluid too quickly and/or not drinking enough fluids) or are taking diuretic medication (water pills) or other blood pressure-lowering medication. If this is the reason, then drinking more fluid or adjusting your medication should help.
Some have a problem with their body's ability to regulate the blood pressure. There are strategies, and several medications, that can be used to ameliorate the severity and symptoms of low pressure in these people.
Keeping your blood pressure in range is an important determinant of long term health. If yours is out of range, either too high or too low, talk to your health care professional.