What Causes Low Blood Pressure?
Low blood pressure, or hypotension, occurs when the pressure of blood circulating in your body is lower than the expected range, typically below 90/60mmHg. While often a sign of good health in fit individuals, it can sometimes be caused by underlying medical issues, lifestyle factors, or specific environmental triggers. Understanding the root cause is essential for determining whether the condition requires management. This article outlines the primary causes of low blood pressure, explains common triggers, and provides guidance on when symptoms might indicate a need for a medical review, all aligned with UK clinical standards.
What We’ll Discuss in This Article
- The biological mechanisms that lead to a drop in blood pressure.
- Lifestyle factors and temporary conditions like dehydration or pregnancy.
- Medical conditions including heart problems and endocrine disorders.
- A detailed look at medications that can cause hypotension as a side effect.
- Common physical triggers that lead to sudden, temporary drops.
- How to differentiate between various types of hypotension.
- Safety advice and when to seek emergency help.
Common Causes of Low Blood Pressure
Blood pressure is determined by the amount of blood your heart pumps and the amount of resistance to blood flow in your arteries. A drop in pressure usually happens because the volume of blood decreases, the heart is not pumping effectively, or the blood vessels have widened too much. In the UK, one of the most frequent causes is simple dehydration. When the body loses more fluid than it takes in, blood volume drops, leading to lower pressure.
Other significant causes include:
- Pregnancy: During the first 24 weeks of pregnancy, the circulatory system expands rapidly, which commonly causes blood pressure to drop.
- Anaemia: A lack of red blood cells or low levels of Vitamin B12 and folate can prevent the body from maintaining adequate pressure.
- Endocrine Problems: Conditions affecting the thyroid or adrenal glands (such as Addison’s disease) can disrupt the hormones that help regulate blood pressure.
- Heart Conditions: A very slow heart rate (bradycardia), heart valve problems, or heart failure can prevent the heart from circulating enough blood.
| Cause Type | Examples | Impact on Body |
| Lifestyle/Temporary | Dehydration, prolonged bed rest | Lowers total blood volume. |
| Medical Conditions | Anaemia, Thyroid issues, Diabetes | Disrupts fluid balance or heart efficiency. |
| Cardiac Issues | Bradycardia, Heart valve disease | Reduces the heart’s pumping power. |
Medications That Can Cause Hypotension
Many people in the UK take medications that may unintentionally lower their blood pressure. This is particularly common in patients being treated for high blood pressure, where the medication may work ‘too well’, or in those taking drugs for unrelated conditions that have a secondary effect on the vascular system.
Medications often linked to low blood pressure include:
- Diuretics: Often called ‘water pills’, these reduce fluid in the body to treat hypertension.
- Beta-blockers and Alpha-blockers: Used for heart conditions and to relax blood vessels.
- Antidepressants: Certain types, such as tricyclic antidepressants, can cause a drop in pressure.
- Parkinson’s Disease Medications: Drugs like levodopa can affect the body’s ability to regulate pressure when moving.
- Erectile Dysfunction Drugs: Particularly when taken in combination with heart medications like nitroglycerin.
Triggers for Sudden Drops in Blood Pressure
Even if your baseline blood pressure is normal, certain triggers can cause it to drop suddenly and temporarily. These episodes are often related to the nervous system’s response to external stimuli or physical changes. The most common trigger is standing up too quickly, which causes blood to pool in the legs due to gravity.
Frequent triggers include:
- Postural Changes: Moving from a lying or sitting position to standing (orthostatic hypotension).
- Large Meals: Blood is diverted to the digestive tract after eating, which can lower pressure elsewhere (postprandial hypotension).
- Heat Exposure: Hot weather, long hot showers, or saunas cause blood vessels to dilate.
- Physical Straining: Straining on the toilet or lifting heavy objects can stimulate the vagus nerve, causing a drop.
- Emotional Stress: Sudden pain, fear, or unpleasant sights can trigger a ‘vasovagal’ response, leading to fainting.
Differentiation: Types of Hypotension
It is helpful to understand the specific type of hypotension you may be experiencing, as the causes and management strategies differ. While ‘low blood pressure’ is a general term, clinicians often categorise it based on when and why the drop occurs.
| Type of Hypotension | Primary Trigger/Cause | Common Symptoms |
| Orthostatic | Standing up from sitting or lying down. | Brief dizziness or ‘seeing stars’. |
| Postprandial | Occurs 1 to 2 hours after eating a meal. | Dizziness, falls, or fainting. |
| Neurally Mediated | Long periods of standing or emotional stress. | Nausea, dizziness, and fainting. |
| Severe (Shock) | Serious infection, blood loss, or allergy. | Confusion, cold skin, rapid breathing. |
Conclusion
The causes of low blood pressure are diverse, ranging from mild dehydration and pregnancy to complex medical conditions or medication side effects. For most people, identifying the trigger such as standing too quickly or being in a hot environment is the first step toward managing the symptoms. While often harmless, persistent low blood pressure that causes dizziness or falls should be monitored to ensure it isn’t a sign of an underlying issue.
If you experience severe, sudden, or worsening symptoms, call 999 immediately.
Can a lack of vitamins cause low blood pressure?
Yes, a deficiency in Vitamin B12, folate, and iron can lead to anaemia, which prevents your body from producing enough red blood cells, resulting in low blood pressure.
Why does my blood pressure drop after I eat?
This is known as postprandial hypotension. After eating, your body directs large amounts of blood to your digestive tract, which can cause a temporary drop in blood pressure in the rest of your body.
Is low blood pressure a symptom of diabetes?
Diabetes can damage the nerves that help send signals to your blood vessels to regulate pressure, leading to drops, especially when standing.
Can stress or anxiety cause low blood pressure?
While anxiety usually raises blood pressure, certain types of acute emotional shock can trigger a vasovagal response, which suddenly lowers the heart rate and blood pressure.
Is low blood pressure during pregnancy permanent?
No, blood pressure usually returns to the mother’s pre-pregnancy levels shortly after giving birth.
How does dehydration lead to low blood pressure?
When you are dehydrated, your body loses fluid volume. Since blood is largely made of water, a decrease in total body fluid leads to a decrease in blood volume and pressure.
Authority Snapshot (E-E-A-T Block)
This article was written by Dr. Rebecca Fernandez, a UK-trained physician with an MBBS and extensive experience across cardiology, intensive care, and emergency medicine. Dr. Fernandez has a background in managing critically ill patients and integrating digital health solutions, ensuring the information provided is clinically accurate and safe for the public. This guide aims to explain the complex causes of hypotension clearly, following NHS and NICE frameworks for cardiovascular health.
