Low blood pressure (hypotension) has many possible causes, from lifestyle choices, to medication or an underlying health condition.
In some cases, it may just be the result of being healthy and active or a tendency you've inherited from your parents.
Throughout the day, it's normal for your blood pressure to vary depending on what you're doing. Stress at work, the temperature outside and your diet could all affect your blood pressure reading.
This is why it's important your blood pressure is checked under similar conditions each time to ensure results are consistent.
If your blood pressure reading is low, your GP will first consider whether it has been affected by:
- the time of day – blood pressure is normally lower overnight while you're sleeping, rises a few hours before you wake up, and continues to rise during the day, reaching its highest mid-afternoon
- 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're healthy and exercise regularly, your blood pressure will be low when you're resting
- temperature – a warm temperature may cause your blood pressure to fall
- if you have recently eaten – blood is diverted to the gut when food needs to be digested, so the blood pressure elsewhere in your body falls
If your blood pressure is still considered low after taking these factors into account, there may be another cause. Some possibilities are explained below.
Some research has suggested low blood pressure is genetic. If your parents have low blood pressure, it's possible you could inherit it from them.
Your blood pressure usually increases as you get older, but a drop in blood pressure from movement or eating is more common with age.
Some types of medication may cause low blood pressure, including the following:
- beta-blockers – a medicine that may be prescribed for a problem with your heart
- angiotensin-converting enzyme (ACE) inhibitors
- angiotensin receptor blockers
- alpha-blockers – medicine prescribed to lower blood pressure for people with high blood pressure (hypertension) and men with prostate gland problems
- diuretics (water tablets)
- some antidepressants
Your GP will discuss possible side effects with you when prescribing medication, and your blood pressure will be carefully monitored if you're considered to be at risk of hypotension.
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.
An illness or underlying health condition
Examples of conditions and illnesses that can lead to low blood pressure are given below. Prolonged bed rest can also lower blood pressure.
Low blood pressure can sometimes be caused by anaemia, where the amount of haemoglobin in the blood is below the normal level or there are fewer red blood cells than normal.
A heart condition, such as heart disease or a heart attack, can cause low blood pressure because your heart may not be able to pump blood around your body.
Neurological disorders, such as Parkinson's disease, are conditions that affect your nerves. Low blood pressure can occur if part of your nervous system called the autonomic nervous system is affected.
Your autonomic nervous system controls bodily functions you don't actively think about, such as sweating and digestion. It also controls the widening and narrowing of your blood vessels.
If there's a problem with your autonomic nervous system, your blood vessels could remain too wide, which can cause low blood pressure.
Having a condition that affects the production of certain hormones in your body can also cause low blood pressure. Examples are diabetes and Addison's disease.
In Addison's disease, the immune system attacks and damages the adrenal glands, two small glands above your kidneys that produce hormones to 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 tumour.
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.
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 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, 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 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.