Low blood sugar, also called hypoglycaemia or a "hypo", is one of the most common short-term problems for people treating diabetes with insulin or certain tablets. Most hypos are mild and can be treated at home in a few minutes, if you and the people around you know the signs and keep something sugary within reach.

The NHS defines a hypo as a blood sugar (glucose) level that is usually below 4 mmol/L. In countries that use mg/dL, the US Centers for Disease Control and Prevention (CDC) says blood sugar below 70 mg/dL is considered low. This guide explains the symptoms, the steps to treat a hypo, which medicines carry the most risk, and when a hypo is an emergency.

What counts as low blood sugar?

Different guidelines describe the same thing in different units:

Source	Low blood sugar	More serious
NHS (UK)	Usually below 4 mmol/L	Seizure or unconsciousness
CDC (US)	Below 70 mg/dL	Below 54 to 55 mg/dL is "severely low"
American Diabetes Association, 2025	Level 1: below 70 but at least 54 mg/dL	Level 2: below 54 mg/dL. Level 3: confusion or physical problems needing someone else's help, at any glucose level

Glucose meters and laboratory reports use either mmol/L or mg/dL. Check which unit yours uses before you compare a reading with any guide, because the numbers are very different.

The American Diabetes Association's Level 3 matters most in practice. A hypo is severe when the person cannot treat it themselves, whatever the number on the meter.

Symptoms of low blood sugar

Early warning signs

The NHS lists these early symptoms:

feeling hungry
sweating
feeling shaky
feeling dizzy
feeling anxious or irritable
tingling lips
palpitations (a fast or pounding heartbeat)
feeling tired or weak
blurred vision
feeling confused

The CDC adds nervousness and "brain fog".

Severe signs

As blood sugar falls further, the CDC says a person may feel weak, have trouble walking or seeing clearly, act strangely or become confused, or have a seizure (fit). They can also become unconscious.

Hypos at night

The CDC says some people have low blood sugar while they sleep, and that you may wake up when it happens, "but you shouldn't rely on that." Its listed reasons for night-time lows include drinking alcohol at night and being active close to bedtime.

The NHS advises seeing your doctor if you have had hypos at night. If your morning readings are puzzling, our guide to high fasting blood sugar in the morning looks at whether a night-time low could be part of the picture.

When you do not feel the warning signs

Some people have no symptoms when their blood sugar is low. This is called hypoglycaemia unawareness. The CDC says it is more likely if you have had diabetes for more than 5 to 10 years, if you often have low blood sugar, or if you take certain medicines such as beta blockers for high blood pressure.

If this applies to you, the CDC advises checking your blood sugar more often, and the NHS advises seeing your doctor.

How to treat a hypo: step by step

The steps below are taken from the NHS and CDC. If your diabetes team has given you a personal hypo plan, follow that.

Check your blood sugar if you can.
Have something sugary that works fast. The NHS gives these examples: a small glass of fruit juice or sugary fizzy drink, "5 glucose or dextrose tablets, 4 large jelly babies, or 2 tubes of glucose gel." The CDC's version, called the 15-15 rule, is to "have 15 grams of carbs, then wait 15 minutes," and lists options such as "4 ounces (½ cup) of juice or regular (non-diet) soda" and "1 tablespoon of sugar, honey, or syrup."
Wait and recheck. The NHS says to check your blood sugar after 10 to 15 minutes. If it is still below 4 mmol/L, have another sugary drink or snack and check again after 10 minutes.
Repeat until you are back in range. The CDC says to keep repeating the steps until your blood sugar is back in your target range.
Then eat something more substantial. Once you feel better and your reading is above 4 mmol/L, the NHS suggests something that keeps your sugar up for longer, such as biscuits, a sandwich or your next meal. The CDC suggests a balanced snack or meal with protein and carbohydrates.
Write it down. Note the time, the reading, what you had eaten and any reason you can think of. Patterns help your prescriber adjust your treatment.

If glucose tablets or gels are hard to get or afford where you live, fruit juice, a non-diet fizzy drink, or sugar or honey are widely available and are among the options the NHS and CDC list. Keep one with you in a bag, at work and beside the bed.

If someone is very drowsy or unconscious

The NHS advice for helping someone who is unconscious is:

Do not give them any food or drink, because they will not be able to swallow safely.
Put them in the recovery position.
Give a glucagon injection straight away if one is available and you know how to use it.
Stay with them until they have fully recovered. If they start to recover within 10 minutes of having a glucagon injection and can swallow safely, give them some food or drink that will raise their blood sugar.

The CDC notes that glucagon is a prescription emergency medicine. If you or a family member takes insulin, ask your clinic whether glucagon is available to you and who in your household should learn to use it.

What causes low blood sugar in diabetes?

Medicines

According to the NHS, insulin and some other diabetes medicines, such as gliclazide and glimepiride, can lower blood sugar too far. These tablets belong to a group called sulphonylureas.

A CPD article for pharmacists in South African Family Practice says hypoglycaemic episodes "have been reported with all the sulphonylureas", and that glimepiride and glipizide are associated with less hypoglycaemia than glibenclamide.
Glibenclamide (glyburide) carries the highest risk in this group. The World Health Organization removed it from its Model List of Essential Medicines in 2013, chose gliclazide as the replacement, and noted that glibenclamide "should not be used in patients aged 60 years and older". A 2007 meta-analysis cited in that decision found an 83% higher risk of hypoglycaemia with glibenclamide than with other sulphonylureas.
Singapore's Health Sciences Authority advises avoiding glibenclamide in older people and those with kidney problems, because they are "more susceptible to severe and recurrent hypoglycaemia". The South African Family Practice article gives the advice to stop glibenclamide when kidney function (eGFR) falls below 60.

If you are on glibenclamide and are over 60, or have kidney problems, ask your prescriber whether it is still the right tablet for you. Do not stop or swap it yourself.

Metformin behaves differently. Gateshead Health NHS Foundation Trust advises that metformin "does not usually cause hypoglycaemia (low blood sugars) when taken on its own". The words "on its own" matter: if you also take insulin or a sulphonylurea, those medicines can still cause hypos. Our guide to metformin side effects covers this in more detail.

GLP-1 medicines such as semaglutide (Ozempic, Rybelsus): Diabetes UK says hypos are more likely when semaglutide is taken "with other diabetes medications such as insulin or a sulphonylurea", and that your team may reduce the dose of your sulphonylurea or insulin when you start it. See our GLP-1 side effects guide.

Everyday triggers

The NHS and CDC list these common triggers:

missing or delaying a meal, or not eating enough carbohydrate
more physical activity than usual without extra carbohydrate or a change in medicine
drinking alcohol, especially without food
taking too much insulin, or injecting into the same spot too often
hot and humid weather, and unexpected changes to your routine

The American Diabetes Association also lists kidney disease, dementia, previous hypos and alcohol as risk factors.

Fasting for religious reasons, long journeys, and night shifts can all disrupt meals. If you plan to fast, speak to your prescriber well beforehand about whether your medicines need adjusting.

Low blood sugar without diabetes

This is rare. The NHS lists malnutrition, Addison's disease and, rarely, a growth in the pancreas among the possible causes. If you get shaking, sweating, a fast heartbeat or confusion and you do not have diabetes, see a doctor.

How to lower your risk of hypos

Know your medicines. Ask your pharmacist: "Can this tablet cause low blood sugar?" If the answer is yes, keep a fast-acting sugar with you at all times.
Do not skip meals on days you take insulin or a sulphonylurea. If you cannot eat, for example because you are unwell, ask your clinic what to do with your medicine. Do not guess.
Check before you drive or exercise. The CDC advises checking your blood sugar before driving or being physically active, especially if you do not always feel the warning signs.
Eat when you drink. The CDC notes that eating when you drink alcohol can help.
Tell the people around you. The NHS advises making sure family and friends know how to treat severe low blood sugar.
Carry identification. The NHS suggests carrying medical ID, and glucagon if you have been prescribed it.
Report every severe hypo, and frequent mild ones. The NHS advises seeing your doctor if you get hypos often, have had a severe hypo or hypos at night, or do not always recognise the symptoms. Your medicine may need changing.

Good food choices also help keep your blood sugar steady between meals. Our type 2 diabetes food list includes African staples such as sadza and ugali.

I am working on a fuller guide, The Type 2 Diabetes Food List and Meal Plan, which will include a clear explanation of how common diabetes medicines work. See the books page for updates.

Questions to ask your doctor, nurse or pharmacist

Can any of my medicines cause low blood sugar?
What reading should I treat as a hypo, and what should I take?
Should I be prescribed glucagon, and who in my family should learn to use it?
I am over 60 (or have kidney problems). Is glibenclamide still right for me?
What should I do with my diabetes medicines on days I cannot eat?

Frequently asked questions

What blood sugar level is too low?

The NHS defines a hypo as blood sugar usually below 4 mmol/L. The CDC uses below 70 mg/dL, and calls below about 54 mg/dL severely low. A hypo is severe if the person needs someone else's help, whatever the reading.

What is the fastest way to raise low blood sugar?

Something sugary and low in fat, such as fruit juice, a non-diet fizzy drink, glucose tablets or glucose gel. The NHS advises rechecking after 10 to 15 minutes and repeating if your level is still below 4 mmol/L.

Can metformin cause low blood sugar?

Not usually on its own. Gateshead Health NHS Foundation Trust advises that metformin does not usually cause hypos when taken alone. If you also take insulin or a sulphonylurea such as gliclazide, those medicines can still cause hypos.

Can you have low blood sugar without symptoms?

Yes. This is called hypoglycaemia unawareness. The CDC says it is more likely after 5 to 10 years with diabetes, with frequent lows, or with medicines such as beta blockers. Check your blood sugar more often and tell your clinician.

Why should I not eat chocolate to treat a hypo?

Chocolate is high in fat, and the CDC explains that fat slows down how fast you absorb sugar. Use a fast-acting sugar first, then eat a longer-lasting snack once you have recovered.

When is low blood sugar an emergency?

When the person is unconscious, having a seizure, or too confused to swallow safely. Do not put food or drink in their mouth. Put them in the recovery position, give glucagon if available and you are trained, and get emergency help.

This article is general information and does not replace advice from your own doctor, nurse or pharmacist. Please do not stop or change any diabetes medicine without speaking to them first.

Sources

NHS. Low blood sugar (hypoglycaemia).
NHS. Hypoglycaemia (hypos), type 1 diabetes.
Centers for Disease Control and Prevention. Treatment of low blood sugar (hypoglycemia). Last reviewed 15 May 2024.
Centers for Disease Control and Prevention. Low blood sugar (hypoglycemia). Last reviewed 16 May 2024.
American Diabetes Association. Hypoglycemia guidelines (based on the Standards of Care in Diabetes, 2025).
World Health Organization. Model List of Essential Medicines: glibenclamide.
Health Sciences Authority, Singapore. Recommendations to avoid use of glibenclamide in the elderly and renal-impaired. 31 December 2013.
Mkele G. What's the latest on sulphonylureas in the management of type 2 diabetes? S Afr Fam Pract. 2013;55(6S):501-503.
Gateshead Health NHS Foundation Trust. Metformin.
Diabetes UK. Semaglutide.