Blood pressure (low) - Causes of low blood pressure

Throughout the day, it is normal for your blood pressure to vary depending on what you are doing.

Stress at work, the temperature outside and your diet could all affect your blood pressure reading.

Each time you have your blood pressure measured, it's important the test is carried out under similar conditions to ensure results are consistent. If you have a low blood pressure reading, your GP will first consider everyday causes that might have affected it, before considering possible underlying causes. 

Everyday causes

Many factors have a daily, or even hourly, effect on your heart and circulation. Below are things that could affect your blood pressure and, in some cases, may cause low blood pressure.

• The time of day - your blood pressure falls overnight and rises during the day.
• Your age - your blood pressure usually increases as you get older, but a drop in blood pressure from movement or eating is more common with age.
• How stressed or relaxed you are - you have lower blood pressure the more relaxed you are. 
• How much exercise you do - initially, exercise will raise your blood pressure, but if you are healthy and exercise regularly, your blood pressure will be low when you are resting.
• Temperature - a warm temperature may cause your blood pressure to fall.
• If you have recently eaten - blood will be used for digesting food in your stomach, so the blood pressure elsewhere in your body will fall.

Underlying causes

If your blood pressure is still considered low after taking into account everyday factors, such as those listed above, there may be another cause. Some possibilities are explained below.

Medication

Some types of medication may cause low blood pressure, including the following:

• blood pressure lowering medication
• beta-blockers - medicine that may be prescribed for a problem with your heart
• alpha-blockers - medicine prescribed to lower blood pressure for people with high blood pressure (hypertension) and those with prostate gland problems (the prostate is a small gland only found in men located between the penis and bladder) 
• some antidepressants

Your GP will discuss possible side effects with you when prescribing medication and your blood pressure will be carefully monitored if you are considered to be at risk of hypotension.

Dehydration

Dehydration can occur if fluid is lost, either through skin from excessive sweating in hot weather, or from the gastrointestinal tract as a result of vomiting or diarrhoea.

Serious illness or conditions

If you have a short-term (acute) illness, your blood pressure will be measured regularly as it is a good indicator of how severe your illness is. A heart condition, such as heart disease or a heart attack, can also cause low blood pressure because your heart may not be able to pump blood around your body.

Neurological disorders

Neurological disorders, such as Parkinson’s disease, are conditions affecting your nerves. Low blood pressure can occur if part of your nervous system called the autonomic nervous system is affected.

Your autonomic nervous system is the part of your nervous system that controls bodily functions you do not actively think about, such as sweating and digestion. It also controls the widening and narrowing of your blood vessels.

If there is a problem with your autonomic nervous system, your blood vessels could remain too wide, and this can cause low blood pressure.

Hormone problems

Having a condition that affects production of certain hormones in your body, such as diabetes or Addison's disease, can also cause low blood pressure.

Addison's disease is where the immune system attacks and damages the adrenal glands, which are two small glands located just above your kidneys. They produce hormones that control your blood pressure and maintain the balance of salt and water in your body.

Low blood pressure can also occur if your adrenal glands become damaged, for example because of an infection or a tumour.  

Serious injury and shock

Low blood pressure can be caused by serious injuries or burns, particularly if you lose a lot of blood. Low blood pressure can also occur if you go into shock after having a serious injury.

Septic shock and toxic shock syndrome

Septic shock and toxic shock syndrome are caused by bacterial infections. Bacteria attack the walls of the small blood vessels, causing them to leak fluid from the blood into the surrounding tissues. This causes a significant drop in blood pressure.

Anaphylactic shock

Anaphylactic shock, or anaphylaxis, is caused by an allergic reaction. During an allergic reaction, your body produces a large amount of a chemical called histamine, which causes your blood vessels to widen, leading to a sudden, severe drop in blood pressure.

Cardiogenic shock

Cardiogenic shock occurs when your heart cannot supply enough blood to your body, leading to a drop in blood pressure. This can happen during a heart attack.

Other causes

Other possible causes of low blood pressure are listed below.

• miscommunication between the heart and brain - low blood pressure that occurs after standing for long periods of time (neurally mediated hypotension) happens when your body tells the brain your blood pressure is too high, when it's actually too low; this causes your brain to slow down the heartbeat, further reducing your blood pressure
• anaemia - a condition where the amount of haemoglobin in the blood is below the normal level, or there are fewer red blood cells than normal
• prolonged bed rest 
• your genes - some research has suggested that low blood pressure is genetic. If your parents have low blood pressure, it is possible you could inherit it from them

In some cases, there is no obvious cause of low blood pressure.