If your blood pressure drops below 90/60, then this can be considered to be low blood pressure but only one of those numbers would need to be below this for example, 100/50. However, everyone is different and there are a number of factors that could affect your blood pressure for example, are you fit or regularly exercise? Do you smoke? If you are a very fit person, then sometimes your blood pressure can drop below the average and you can be absolutely fine.
Hypertension is the condition of high blood pressure. The opposite of this is Hypotension, when the blood pressure is extremely low. Low blood pressure can be caused by hormonal changes, a side effect of medicine, anaemia or a widening of the blood vessels. Hypovolemia (reduced blood volume) is the most common cause of low blood pressure which can result from a haemorrhage or blood loss, not taking in enough fluids or an excessive loss of fluids.
However, as mentioned before, your blood pressure can drop below the healthy boundary and you can be fine. This is because you are only diagnosed as having low blood pressure if you show the symptoms as well as a blood pressure below 90/60. Symptoms of low blood pressure include: Chest pain, shortness of breath, irregular heartbeat, a fever higher than 38.3 degrees C (101 degrees F), headache, stiff neck, severe upper back pain, coughing with phlegm, prolonged diarrhea or vomiting, dyspepsia (indigestion), dysuria (painful urination), foul smelling urine, seizures, loss of consciousness, fatigue, temporary blurring or loss of vision and in some cases loss of hair.