Low blood sugar, or hypoglycemia, usually means a glucose reading below 70 mg/dL. It can come on quickly, but most episodes are easy to treat if you know the signs and keep fast-acting carbohydrate within reach. This guide covers what a low feels like, the widely taught "15-15" approach, and when a low becomes an emergency.
Key takeaways
- For many people with diabetes, a reading under 70 mg/dL counts as low. Your own number may differ, so confirm it with your care team.
- Lows are most common in people who use insulin or medicines such as sulfonylureas and meglitinides.
- Standard advice from the NIDDK is 15 to 20 grams of fast-acting carbohydrate, then a recheck after 15 minutes.
- A severe low, where you cannot treat yourself, is an emergency. Glucagon and a call to emergency services are the recognised response.
What counts as low blood sugar?
The US National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) describes low blood glucose as a level that drops below what is healthy for you, which for many people with diabetes means under 70 mg/dL (3.9 mmol/L). The American Diabetes Association (ADA) groups lows into three levels:
| Level | What it means |
|---|---|
| Level 1 | Below 70 mg/dL but at or above 54 mg/dL |
| Level 2 | Below 54 mg/dL (3.0 mmol/L) |
| Level 3 | A severe event where you need another person's help, whatever the number |
Level 3 is defined by what is happening to you, not by a reading. If you are too confused or drowsy to treat yourself, it is severe.
Who is most at risk?
Lows are mainly a side effect of treatment rather than of diabetes itself. According to the NIDDK, you are more likely to have them if you have type 1 diabetes, take insulin or certain tablets that prompt the pancreas to release insulin (sulfonylureas and meglitinides), are aged 65 or older, have had lows before, or live with kidney disease, heart disease or cognitive impairment.
They are also common. In a large global study of people using insulin, about 4 in 5 people with type 1 diabetes and nearly half of those with type 2 reported at least one low over a four-week period.
Everyday triggers include:
- Skipping or delaying a meal, or eating fewer carbohydrates than usual
- More physical activity than normal, which can lower glucose for up to 24 hours afterwards
- Drinking alcohol without enough food (see our guide to alcohol and diabetes)
- Illness that makes it hard to eat or keep food down (see the sick day guide)
- Fasting while still taking glucose-lowering medicine
How to spot a low
Symptoms tend to arrive fast and differ from person to person. The NIDDK lists these common mild-to-moderate signs:
- Feeling shaky or jittery
- Hunger
- Tiredness
- Dizziness, lightheadedness, confusion or irritability
- A fast or uneven heartbeat
- Headache
- Trouble seeing or speaking clearly
When glucose falls very low, the brain cannot work as it should. Loss of consciousness and seizures are signs of a severe low and need immediate help.
Lows during sleep
Glucose can drop overnight and stay low for hours. Clues include nightmares or crying out, sweating enough to dampen sheets or pajamas, and waking up tired, irritable or confused.
When you stop feeling the warning signs
Repeated lows can lead to hypoglycemia unawareness, where you notice no symptoms until glucose is already very low. If this sounds familiar, tell your care team. The NIDDK notes that a continuous glucose monitor (CGM) with alarms may be a good option for people who have unawareness or frequent lows. Our CGM basics guide explains how sensors and alerts work.
The 15-15 approach to treating a low
If you feel symptoms, check your glucose if you can. If it is below your target or under 70 mg/dL, the steps taught by the NIDDK and the CDC are:
- Take 15 to 20 grams of fast-acting carbohydrate.
- Wait 15 minutes and check again.
- Repeat if you are still low, until you are back in your target range.
- Have a snack, such as crackers or a piece of fruit, if your next meal is more than an hour away.
Examples of 15 to 20 grams of fast-acting carbohydrate from the NIDDK:
- Four glucose tablets or one tube of glucose gel
- Half a cup (4 ounces) of regular fruit juice
- Half a can (4 to 6 ounces) of regular, not diet, soda
- One tablespoon of sugar, honey or corn syrup
Two details are worth knowing. The NIDDK advises people with kidney disease to avoid orange juice because of its potassium content; apple, grape or cranberry juice are alternatives. And if you take a medicine that slows the digestion of carbohydrates, glucose tablets or gel are needed, because other foods and drinks will not raise glucose quickly enough. Ask your pharmacist or doctor whether this applies to you.
The amounts above are general public-health guidance for adults. Children and some adults may be given different amounts, so follow the plan your own care team has set.
Severe lows: when it is an emergency
A severe low is one you cannot treat yourself. The person may be unable to swallow safely, may be unconscious or may be having a seizure. The CDC describes a reading below 55 mg/dL as severely low and warns that the 15-15 steps may not be enough at that point.
Glucagon, a hormone that raises blood glucose, is the recognised treatment. It is available by prescription as an injection or a nasal spray. The ADA's message to bystanders is clear: if someone is unconscious and glucagon is not available, or no one knows how to use it, call emergency services immediately. The NIDDK also advises calling 911 (or your local emergency number) straight after glucagon is given.
Being prepared matters more than anything you can do in the moment:
- Ask your doctor whether you should have glucagon, and how and when it should be used.
- Check the expiry date on the pack from time to time.
- Show family, friends and coworkers where it is kept and how to give it.
- Consider wearing a medical alert bracelet or pendant.
Preventing the next one
You cannot avoid every low, but patterns help. Simple habits that the NIDDK recommends include:
- Carry fast-acting carbohydrate everywhere: glucose tablets or a juice box in your bag, car, desk and gym kit.
- Check around exercise. Activity can lower glucose during the session and for hours afterwards, so you may need to check before, during and after.
- Eat regularly and include food when you drink alcohol.
- Review your medicines. Ask your team which of yours can cause lows.
If you train with DiaFitus, the app's pre-workout glucose check prompts you to enter a reading before a session and reminds you to keep fast sugar nearby. It is a planning aid only. It does not replace your meter, your CGM alarms or medical advice.
When to talk to your care team
Bring it up at your next appointment, or sooner, if you have lows often, have had any low below 54 mg/dL, needed someone else's help, have lows at night, or no longer feel the early symptoms. The ADA's guidance to clinicians is that these events are a reason to review the treatment plan. Never change or stop a medicine on your own because of lows; your team can adjust things safely.
Fear of lows is also worth mentioning. The NIDDK points out that worry about hypoglycemia can lead people to run their glucose higher than is healthy. Having a clear plan, and people around you who know it, tends to make lows less frightening.
The bottom line
Know your symptoms, keep fast-acting carbohydrate close, treat promptly and recheck after 15 minutes. Make sure the people around you know what a severe low looks like and what to do. This article is general education, not personal medical advice, so use it as a starting point for a conversation with your doctor or diabetes care team.
