What Is Low Blood Sugar? A UK Guide to Hypoglycaemia
What is low blood sugar? For people with diabetes, low blood sugar is usually a blood glucose level below 4 mmol/L. Common symptoms include shaking, sweating, hunger, dizziness, palpitations, tingling lips, irritability and confusion. It should be treated promptly according to your diabetes plan because symptoms can worsen if glucose continues to fall.
Low blood sugar at a glance
| Question | Short answer | Why it matters |
|---|---|---|
| What number is low? | Usually below 4 mmol/L in UK diabetes guidance. | A reading below this level should usually be treated promptly if you have diabetes. |
| Who gets hypos? | Most commonly people using insulin or some other diabetes medicines. | Not everyone with diabetes is at the same risk. |
| How fast can it happen? | Symptoms can appear quickly. | Carry your preferred treatment rather than relying on finding something later. |
| Is it always obvious? | No. Symptoms differ, and some people have reduced hypo awareness. | Glucose monitoring can be important even when symptoms are mild or absent. |
What does a diabetic hypo feel like?
There is no single feeling that everyone experiences. A hypo may begin with physical warning signs such as shaking, sweating, hunger, a racing heartbeat or tingling lips. Other people first notice mental or emotional changes such as anxiety, irritability, poor concentration or confusion.
As glucose falls further, a person may become drowsy, disorientated or unable to treat themselves. Seizures and unconsciousness can occur in severe hypoglycaemia. For a more detailed symptom-by-symptom guide, see our low blood sugar symptoms guide.
Why does low blood sugar happen?
A hypo usually develops when there is more glucose-lowering effect than the body needs at that moment. The exact trigger varies, but common situations for people with diabetes include:
If you do not have diabetes, true hypoglycaemia is uncommon. Repeated low readings or hypo-like symptoms should be assessed rather than self-diagnosed. Read our guide to low blood sugar without diabetes.
What to do if your blood sugar is below 4 mmol/L
If you have diabetes and your glucose is below 4 mmol/L, or you have typical hypo symptoms, follow the treatment plan agreed with your diabetes team. NHS and Diabetes UK guidance uses the following general approach for adults:
Severe hypo: when another person needs to act
If someone is unconscious, having a seizure or cannot swallow safely, do not give food or drink by mouth. Put them in the recovery position, use prescribed glucagon if available and you know how to use it, and call emergency services when required. Severe hypoglycaemia is defined by the need for assistance, not simply by one glucose number.
Fast-acting carbohydrate options
| Format | Practical advantage | What to check |
|---|---|---|
| Glucose tablets | Compact and easy to carry. | Carbohydrate per tablet and how many are needed for your plan. |
| Glucose shot | Drinkable and can be pre-measured. | Total carbohydrate per bottle. |
| Glucose gel | Soft format with no chewing. | Carbohydrate per tube or sachet. |
| Sugary drink or juice | Often familiar and easy to drink. | Recipes can change; make sure the drink contains sugar rather than being diet or sugar-free. |
For a deeper explanation of the category, read What Is Fast-Acting Glucose?
Frequently asked questions
What is considered low blood sugar in the UK?
For people with diabetes, NHS and Diabetes UK guidance usually treats blood glucose below 4 mmol/L as a hypo.
What does a diabetic hypo feel like?
Common symptoms include shaking, sweating, hunger, dizziness, palpitations, tingling lips, irritability, tiredness and confusion. Your personal warning signs may be different.
Can you have a hypo without symptoms?
Yes. Some people develop reduced hypo awareness and may not notice the usual warning signs. Speak to your diabetes team if this happens.
What should I take for low blood sugar?
Follow your individual plan. Diabetes UK commonly recommends 15–20g of fast-acting carbohydrate for adults, followed by a recheck after 10–15 minutes.
Is 3.8 mmol/L low blood sugar?
Yes. In UK diabetes guidance, 3.8 mmol/L is below the usual 4 mmol/L hypo threshold. Follow your treatment plan and recheck.
Can people without diabetes have low blood sugar?
They can, but true non-diabetic hypoglycaemia is uncommon. Repeated or unexplained low readings should be medically assessed.