Low Blood Glucose Levels in Diabetes: A Guide

Low blood glucose levels in diabetes, known as hypoglycaemia or a hypo, occur when blood glucose falls to 4 mmol/L or below. For people living with diabetes, understanding why hypos happen, how to recognise the warning signs, and what to do when blood sugar drops too low can make a significant difference to safety and quality of life. This article covers the common causes of hypoglycaemia, the symptoms to watch for, the correct treatment steps, and which diabetes medicines carry the greatest risk, all in line with current NHS and NICE guidance.

What causes low blood glucose levels in people with diabetes?

Hypoglycaemia is most commonly caused by too much insulin or sulphonylurea medication relative to carbohydrate intake, but missed meals, intense exercise, alcohol and impaired hypoglycaemia awareness are also important triggers.

Low blood glucose levels in people with diabetes, commonly called a hypo or hypoglycaemia, occur when blood glucose falls to 4 mmol/L or below, as defined by NHS guidance. This threshold matters because the brain relies almost entirely on glucose as its fuel, and levels at or below this point can impair normal function within minutes.

Several factors can tip the balance between glucose supply and demand. The most common causes include:

• Taking too much insulin or certain diabetes medicines relative to the carbohydrate eaten

• Skipping or delaying a meal after taking glucose-lowering medication

• Unplanned or unusually intense physical activity, which increases glucose uptake by muscles

• Drinking alcohol, particularly on an empty stomach, as alcohol suppresses the liver's ability to release stored glucose

The liver normally acts as a glucose reservoir, releasing glycogen stores when blood sugar falls. In people taking insulin or insulin-stimulating medicines, this compensatory mechanism can be overwhelmed, especially if glycogen stores are already depleted by exercise or poor nutrition.

Physical activity deserves particular attention: glucose can continue to fall for several hours after exercise ends, and overnight hypoglycaemia is a recognised risk following strenuous activity later in the day.^[7][8] Planning additional carbohydrate intake and increasing monitoring around exercise can help reduce this risk; a diabetes care team can advise on an individual approach.

Other contributing factors include intercurrent illness (which can raise or lower blood glucose unpredictably, so following NHS sick-day rules is important), changes in injection site (which can alter insulin absorption), and certain medications such as beta-blockers, which may blunt the counter-regulatory hormonal response and mask adrenergic warning symptoms.^[10][11] People who have had diabetes for many years may also develop impaired awareness of hypoglycaemia, meaning early warning signs are no longer reliably felt. Understanding these triggers is the first step in reducing the frequency and severity of hypos.

How do you recognise the symptoms of a hypo?

Early hypo symptoms include sweating, shakiness and palpitations; if untreated, neuroglycopenic symptoms such as confusion, slurred speech and loss of consciousness can follow.

The symptoms of a hypo typically appear in two distinct stages, with early warning signs giving way to more serious neurological effects if blood glucose continues to fall. Recognising the early stage is critical, because acting promptly at this point can prevent a mild episode from becoming a medical emergency.

Early (adrenergic) symptoms, driven by the release of adrenaline as the body attempts to raise blood glucose, include:

• Sweating, clamminess or feeling shaky

• A rapid or pounding heartbeat (palpitations)

• Hunger, nausea or feeling anxious

• Pallor and light-headedness

• Tingling around the lips or fingertips

Later (neuroglycopenic) symptoms, caused by the brain being deprived of glucose, include:

• Difficulty concentrating or confusion

• Slurred speech or unusual behaviour

• Blurred or double vision

• Weakness or drowsiness

• In severe cases, seizures or loss of consciousness

Symptoms vary considerably between individuals, and the same person may experience different symptoms on different occasions. People who have had diabetes for many years, or who have frequent hypos, may develop hypoglycaemia unawareness, in which the early adrenergic warning signs are absent or greatly reduced. This makes regular blood glucose monitoring, or the use of a continuous glucose monitor (CGM) where clinically appropriate, particularly important for this group.

Family members, carers and colleagues should also be aware of the signs, as a person experiencing a severe hypo may not be able to recognise or communicate their own symptoms. NICE guidance (NG17) supports structured education programmes, such as DAFNE (Dose Adjustment For Normal Eating), which is designed for adults with type 1 diabetes, to help people and their support networks identify and respond to hypos effectively.

Driving: Hypoglycaemia has important implications for driving safety. DVLA guidance on diabetes and driving sets out the legal obligations for people who experience hypos, including requirements to check blood glucose before driving and to stop driving after a severe or recurrent episode. People who drive and have diabetes should familiarise themselves with the current DVLA guidance and discuss any concerns with their diabetes care team.

What should you do when your blood sugar drops too low?

A conscious person should immediately take 15 to 20 g of fast-acting glucose, recheck blood glucose after 10 to 15 minutes, and eat a longer-acting carbohydrate snack once levels recover.

When blood glucose falls to 4 mmol/L or below and a person is conscious and able to swallow, the immediate priority is to take fast-acting carbohydrate to raise blood sugar quickly. NHS guidance recommends consuming 15 to 20 g of fast-acting glucose, which can be provided by:

• 5 to 7 glucose tablets (check the pack for the exact amount)

• 150 to 200 ml of a sugary (non-diet) drink, such as orange juice or a cola

• 4 to 5 jelly babies or similar glucose sweets

After taking fast-acting glucose, blood sugar should be rechecked after 10 to 15 minutes. If it remains at 4 mmol/L or below, the fast-acting carbohydrate should be repeated. If blood glucose has not recovered after a second treatment, seek urgent medical advice. Once blood glucose has returned to a safe level, a longer-acting carbohydrate snack, such as a biscuit, a small sandwich or the next scheduled meal, should be eaten to prevent a further drop.

If the person is unconscious, having a seizure, or unable to swallow safely, do not attempt to give food or drink by mouth. Instead, call 999 immediately. A glucagon emergency kit (available on prescription) can be administered by a trained carer or family member while waiting for emergency services. Glucagon stimulates the liver to release stored glucose and is available as an injection or as a nasal powder (Baqsimi), which does not require a needle.^[16] Glucagon may be less effective if glycogen stores are depleted, for example after prolonged fasting or heavy alcohol use; always follow the instructions in the product leaflet and ensure carers are trained in its use.^[6][16]

After any severe hypo, the person should contact their GP or diabetes care team to review their treatment plan. Frequent hypos may indicate that medication doses, meal timing or monitoring routines need adjustment. Suspected adverse effects from diabetes medicines can be reported to the MHRA via the Yellow Card scheme at yellowcard.mhra.gov.uk.

Which diabetes medicines are most likely to cause hypoglycaemia?

Insulin and sulphonylureas such as gliclazide carry the highest hypo risk; metformin, GLP-1 receptor agonists, SGLT-2 inhibitors and DPP-4 inhibitors have a low intrinsic risk when used alone.

Not all diabetes medicines carry the same risk of causing low blood glucose levels, and understanding which treatments are most likely to trigger a hypo helps people with diabetes and their clinicians make informed decisions about management. The medicines most commonly associated with hypoglycaemia are those that stimulate insulin secretion or replace insulin directly, regardless of the prevailing blood glucose level.

Insulin (all types, including rapid-acting, long-acting and mixed formulations) carries the highest risk, particularly if doses are not carefully matched to carbohydrate intake and activity levels. Sulphonylureas, such as gliclazide and glibenclamide, stimulate the pancreas to release insulin continuously and are the oral medicines most associated with hypos.^[17][18] NICE guidance (NG28) notes that sulphonylureas should be used with caution in older people and those with renal impairment, as the risk of prolonged or severe hypoglycaemia is increased in these groups.^[18] Meglitinides, such as repaglinide, act in a similar way to sulphonylureas and also carry a meaningful risk of hypoglycaemia, particularly if meals are delayed or missed.^[21][22]

By contrast, several widely used diabetes medicines have a low intrinsic risk of hypoglycaemia when used alone:

• Metformin does not stimulate insulin release and rarely causes hypos on its own

• GLP-1 receptor agonists (such as semaglutide and liraglutide) work in a glucose-dependent manner, meaning they reduce their effect as blood glucose falls^[20]

• SGLT-2 inhibitors (such as dapagliflozin and empagliflozin) act on the kidneys and do not directly stimulate insulin

• DPP-4 inhibitors (such as sitagliptin) also have a glucose-dependent mechanism and a low standalone hypo risk

However, combining any of these medicines with insulin or a sulphonylurea increases overall hypo risk. Anyone starting a new diabetes medicine or changing their dose should discuss hypoglycaemia risk with their GP or diabetes specialist. Online consultations, such as those offered by Bolt Pharmacy, a GPhC-registered UK online pharmacy, can provide prescriber review and pharmacist support for people managing their diabetes treatment. Dose adjustments should always be made under clinical supervision, and people should never stop or alter their diabetes medication without professional advice.

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