Low blood sugar can affect anyone. However, it’s more likely if you have diabetes and are taking medicines such as insulin, which lowers your blood sugar levels.

It can also affect you if you have another condition that lowers your blood sugar.

It’s important to recognise the signs of low blood sugar, so you can act quickly. It’s also a good idea to let your friends, family and colleagues know, so they can help you if it happens.

If it’s mild and you treat it quickly, it shouldn’t affect your long-term health. If it’s severe and isn’t treated, it can sometimes be life-threatening.

What is a normal blood sugar level?

Your blood sugar level refers to the amount of glucose (a sugar) in your blood. It is regulated by the hormone insulin, which is produced by your pancreas when your blood sugar level rises. Insulin allows your cells to absorb glucose for use as energy or to store as glycogen. When your blood sugar level drops too low, a hormone called glucagon, which is also produced by your pancreas, is released and causes your liver to break down its glycogen stores into glucose, which is then released into your blood.

Your blood sugar level is measured in millimoles per litre (mmol/L). It is usually maintained at 4–6 mmol/L when you're fasting ie when you haven’t eaten for at least eight hours. When you eat a meal, carbohydrates in your food are broken down into smaller molecules, including glucose. Insulin will be released into your blood to bring your blood sugar levels back down. Around two hours after a meal, if you are healthy, your blood glucose levels can be up to 7.8 mmol/L.

Diabetes is diagnosed according to your blood sugar levels.

How to tell if you have hypoglycaemia

The effects of low blood sugar vary from person to person and can change over time. Early symptoms of low blood sugar include:

Dizziness
Fast heartbeat (palpitations)
Feeling anxious, irritable, moody and/or tearful
Feeling hungry, shaky and/or very tired
Looking paler than usual
Sweating and/or tingly lips

Low blood sugar can develop while you are asleep. This can cause you to wake up in the middle of the night, or when you wake in the morning, you may:

Have a headache
Feel tired
Notice the sheets are damp from sweating

If your blood sugar levels fall very low, you may also experience:

Blurred vision
Collapsing or passing out (unconsciousness)
Confusion and unusual behaviour eg clumsiness or slurred speech (similar to being drunk)
Feeling sleepy
Fits or seizures
Weakness

In very severe cases, low blood sugar can be fatal.

Diagnosis and tests for hypoglycaemia

Your GP will talk to you about your symptoms.

If you’ve already been diagnosed with diabetes, they’ll tell you to see if your medication needs to change or be adjusted.

If you don’t know why your blood sugar levels have been affected, they may carry out some blood tests for diabetes or other medical problems.

Causes of hypoglycaemia

Low blood sugar can be a sign of diabetes or another medical condition that lowers glucose in your blood.

Diabetes

If you’ve been diagnosed with diabetes, it is important to closely monitor your blood sugar levels and learn how to balance your activity levels and the food you eat with your insulin injections or medication.

Not everyone with diabetes needs to have daily insulin injections — it is only needed if you have type 1 diabetes or if you have type 2 diabetes that isn’t well-controlled with tablets.

In type 1 diabetes, your body stops producing insulin, while in type 2 diabetes, your body becomes less responsive to insulin. In either case, this causes your blood sugar levels to rise. Insulin injections or other diabetes medication is needed to lower your blood sugar levels. Taking too much insulin can cause your blood sugar levels to drop too low.

Other causes of hypoglycaemia if you have diabetes include:

Alcohol — this can affect your blood sugar levels, especially if you’re taking insulin
Exercise, particularly if it is unplanned or intense — it’s important to carry some fast-acting carbohydrates (eg glucose tablets) as physical activity lowers your blood sugar levels
Not eating enough carbohydrate (eg bread, cereals, potatoes, pasta or rice) in your last meal
Other medications — this includes diabetes medications that increase insulin production ie glinides (eg repaglinide and nateglinide) and sulfonylureas (eg glibenclamide and gliclazide), as well as certain antiviral medications used to treat hepatitis C
Skipping or delaying meals

Sometimes there is no clear reason why your blood sugar level drops too low.

Other causes

If you do not have diabetes, it is uncommon to develop low blood sugar. However, this may happen if you:

Don’t eat enough, are fasting, malnourished or dieting
Drink too much alcohol without food
Eat too much carbohydrate — eating a very high-carbohydrate meal can trigger reactive hypoglycaemia (also called postprandial hypoglycaemia) where your body releases too much insulin; this can affect anyone but is more common if you have had gastric bypass surgery

Less common causes include:

A complication of pregnancy
Certain medications including quinine (for malaria)
Certain medical conditions eg problems with your hormone levels, adrenal glands, heart, kidneys, liver or pancreas
Weight loss surgery (gastric bypass surgery)

Common treatments for hypoglycaemia

Treatments for low blood sugar will depend on your symptoms. Remember, if your blood sugar is less than 5 mmol/L you should not drive and if it is less than 4 mmol/L, you are having a hypo. The catchphrase ‘five, don’t drive, four is the floor’ is a useful way to remember this.

As symptoms of a hypo can mimic being drunk, make sure that you tell the people around you (friends, family and colleagues) about your diabetes, the signs of a hypo and how they can help. Also make sure you:

Carry an insulin pen or syringe, or glucose testing kit with you
Carry glucose gel or tablets, or sugary sweets with you
Wear a medical alert bracelet or similar

If you think you are having a hypo, sit down and ask someone to assist you by checking your responsiveness, breathing and pulse. Treat your low blood sugar by immediately having a high-sugar snack or drink, such as:

A non-diet fizzy drink
Fruit juice
One or two tubes of glucose gel
Three or six glucose tablets
Two teaspoons of sugar

Test your blood sugar 10-15 minutes after having your sugary snack or drink and if it hasn’t increased at all or much, have another sugary snack or drink and test your blood sugar again.

If your blood sugar has increased enough and you feel better, make sure you then eat a meal or snack containing a slow-release carbohydrate eg bread, toast, a sandwich or a glass of cow’s milk.

If you only have a few hypos and feel better after self-treating by having sugary snacks or drinks, you may not need any more treatment.

However, you should tell your care team as soon as possible if you:

Have a hypo that causes a seizure or fit
Have a hypo where you lose consciousness
Keep having hypos
Notice that you don’t have any symptoms despite your blood sugar being too low

If your hypo is severe, you may need emergency treatment in hospital. This is why it is important for those around you to be aware of the signs of a hypo, so they can call 999 and place you in the recovery position if you become very drowsy or lose consciousness. They will need to ensure your mouth is clear of any objects to prevent you from choking.

If you lose consciousness and have a glucagon injection on you, the person you are with can administer it if they know how — it should be given immediately. If you then regain consciousness, you should eat a slow-release carbohydrate.

What to do if you have a hypo when driving

If you have diabetes, or have any other health condition or take any medication that may cause a hypo, the DVLA recommends checking your blood sugar levels less than two hours before you drive and every two hours thereafter if you will be driving for a long time.

If you notice you are having symptoms of a hypo, pull over as soon as it is safe and take your keys out of the ignition. Check your blood sugar level. If it is less than 5 mmol/L, you shouldn't continue to drive and if it is less than 4 mmol/L you are having a hypo.

Have a sugary snack or drink immediately and if you have someone with you, explain what is going on so they can help if your symptoms worsen. Wait 15 minutes and try to relax. Then check your blood sugar again. If it hasn't risen at all or much, have another sugary snack or drink and check your blood sugar again 15 minutes later.

Once your blood sugar has risen above 5 mmol/L, wait at least 45 minutes before you drive again. If you are due to have a meal by this time, make sure you do and that your meal contains a slow-release carbohydrate.

Hypoglycaemia complications

Untreated hypoglycaemia can cause:

Seizures
Unconsciousness
Death

Hypoglycaemia can also cause:

Dizziness
Falls
Injuries
Motor vehicle accidents
Weakness

It is also linked to a higher risk of dementia in older adults.

How to reduce the risk of hypoglycaemia

If you have type 1 diabetes, the most effective way to reduce your risk of hypoglycaemia is to closely monitor and control your blood sugar levels. This will involve:

Avoiding alcohol, particularly drinking without eating a meal beforehand
Carrying sugary snacks or drinks with you at all times so you can quickly treat hypo symptoms
Learning how to balance your insulin dose with the amount of carbohydrate you eat and your activity levels
Not skipping or delaying snacks or meals
Regularly checking your blood sugar levels — if you have type 1 diabetes it is essential that you do this at least four times a day, including before each meal and before bed; if you have type 2 diabetes, you usually only need to check it yourself regularly if you're taking sulfonylurea tablets or insulin injections
Talking to your care team if:

You're taking sulfonylurea tablets and are concerned that you are getting hypo symptoms — there are alternative medications for type 2 diabetes that don't come with a risk of hypos
You know your normal eating habits will change in the near future so you can adjust your insulin regime eg if you're planning to fast for Ramadan, travel abroad, change your shift work pattern

If you have diabetes, it is also important to keep up with your regular diabetes checks. During these appointments, your average blood sugar levels over the previous three months will be measured using an HbA1c blood test. If the results show that your blood sugar levels are not well-controlled, your care team will advise you on what to do to better manage your diabetes.

Frequently asked questions

What is the main cause of hypoglycaemia?

Hypoglycaemia occurs when your blood sugar levels drop too low. This is most common in people who have diabetes and is often caused by skipping or delaying meals, taking too much insulin, unplanned exercise or exercising intensely, drinking alcohol or not eating enough carbohydrate. In people without diabetes, it can be caused by eating a large, carbohydrate-rich meal, drinking too much alcohol, fasting, dieting or not eating enough.

What does a hypoglycaemic attack feel like?

You may feel dizzy, anxious, irritable, moody or tearful. You may also notice your heartbeat is rapid and feel sweaty, hungry, shaky or very tired. In more severe cases, you may also feel confused and very sleepy or as if you are about to pass out — you may lose consciousness.

What are the symptoms of hypoglycaemia vs hyperglycaemia?

Symptoms of hypoglycaemia tend to develop rapidly within hours, while symptoms of hyperglycaemia develop over days or weeks. Both can cause blurred vision and tiredness, however, hypoglycaemia also makes you feel dizzy, anxious and irritable. Hyperglycaemia can make you feel sick and very thirsty, while hypoglycaemia can make you feel hungry, shaky and sweaty.

With hyperglycaemia, you may need to urinate more frequently, have tummy pain and fruity-smelling breath. You are also more likely to have recurrent infections and unintentional weight loss.

With hypoglycaemia, you may feel confused or very sleepy. In severe cases, you may lose consciousness or have a seizure.

What is BGL hypoglycaemia?

BGL stands for blood glucose level and refers to the level of glucose (a sugar) in your blood. If you have hypoglycaemia, your BGL will be below 4 mmol/L.

How do I know if I'm hypoglycaemic?

A blood test to check your blood sugar levels will give you a clear answer — if your blood sugar levels are less than 4 mmol/L then you are hypoglycaemic. You may notice symptoms such as feeling dizzy, anxious and irritable, as well as feeling hungry, sweating, shaky and very tired or sleepy.

How long do hypoglycaemic attacks last?

A hypoglycaemic attack will persist until you can increase your blood sugar levels. You may be able to do this by simply having a sugary snack or drink. Once you have eaten or drunk enough sugar, your blood sugar levels should rise back up in around 15 minutes.

What are the three classic signs of hyperglycaemia?

Three common signs of hyperglycaemia are feeling thirsty, needing to urinate more frequently and feeling tired. However, you may also have headaches, blurred vision, tummy pain, a fruity-smelling breath and feel sick.

Is hyperglycaemia worse than hypoglycaemia?

Both conditions are serious if left untreated. Hypoglycemia can more quickly become life-threatening if your blood sugar levels aren’t increased. Hyperglycaemia tends to develop over days or weeks and left untreated can cause long-term health problems. In severe cases, hyperglycaemia can also cause life-threatening complications ie diabetic ketoacidosis or hyperosmolar hyperglycaemic state.

How do you prevent hypoglycaemia at night?

Hypoglycaemia at night is most common in people with diabetes who take insulin injections, although it can also occur in people who take other diabetes medications. To help prevent it, test your blood sugar level before bedtime and try to keep it above 6.5 mmol/L before you go to bed. You can do this by making sure that your basal insulin dose is not too high, not skipping dinner or snacks, reducing your long-acting insulin dose in the evening if you have been exercising, and eating some carbohydrate before bedtime if you have been drinking alcohol that evening. You should also speak to your care team, who can help you determine if and how to adjust your insulin doses.

What happens if you ignore hypoglycaemia?

If you ignore hypoglycaemia, it will get worse. Severe hypoglycaemia can cause you to lose consciousness and/or have a seizure. It can even result in death if left untreated.

How do you permanently treat hypoglycaemia?

Hypoglycaemia is caused by your blood sugar levels dropping too low. There isn’t a permanent cure but you can take steps to avoid hypoglycaemia, such as avoiding drinking alcohol on an empty stomach and making sure you eat enough food.

If you have diabetes, you will need to carefully control your blood sugar levels for the rest of your life to avoid both hypoglycaemia and hyperglycaemia. To avoid hypoglycaemia make sure you don’t skip or delay meals or snacks and learn how to regulate your insulin dose or diabetes medication according to the amount of carbohydrate you are eating and your activity levels. Wherever you go, make sure you always have sugary snacks or drinks, glucose gels or glucose tablets with you, in case you do have a hypo.

What foods should be avoided if you have hypoglycaemia?

If you have hypoglycaemia, you do not need to avoid certain foods but rather make sure you eat the right food to bring your blood sugar back up to a healthy level. During a hypo episode, you should eat a sugary snack or drink a non-diet fizzy drink or fruit juice to increase your blood sugar level. Once your blood sugar has risen enough, you should then eat a slow-release carbohydrate snack or meal (eg bread, toast or a glass of cow’s milk) to help maintain your blood sugar level.

Can you live a normal life with hypoglycaemia?

If you are diabetic and frequently have hypoglycaemia, you should speak to your care team as you may need to make changes to better control your blood sugar levels. If your hypos are mild and infrequent, there is no cause for concern; as long as you treat them quickly, you can continue with your normal activities. If you are not diabetic, it isn’t common to have hypoglycaemia and if you do have an episode, it is usually easily treated and will not affect your ability to continue with your usual activities.