Low blood sugar, also called hypoglycaemia, happens when the glucose level in your blood drops too low to fuel your body properly. It can affect anyone, but it is especially common in people living with diabetes. Understanding the warning signs and knowing what to do can make a real difference to your health and safety.
What Is Low Blood Sugar?
Blood sugar, or blood glucose, is the main source of energy for your brain and body. Normal fasting blood sugar sits between 65 and 100 mg/dL (3.6–5.6 mmol/L) after about 12 hours without eating. When levels fall below that range, your body starts sending out warning signals.
Doctors use what is called the Whipple Triad to confirm a diagnosis of low blood sugar. This means three things must be present: symptoms that match hypoglycaemia, a measured blood sugar of 40 mg/dL (2.2 mmol/L) or lower during those symptoms, and clear improvement once blood sugar returns to normal.
It is worth knowing how blood sugar is measured. A lab test uses plasma from a vein, while a home glucose meter uses a drop of blood from your fingertip. Fingertip readings can run 10–15% lower than vein plasma results. Home meters also carry a built-in margin of error of about ±15%, so always discuss your readings with your family doctor or diabetes care team.
Common Causes of Low Blood Sugar
Low blood sugar does not always have one simple cause. However, certain situations make it much more likely to occur.
Fasting and Skipping Meals
Going without food for an extended period is one of the most common triggers. Interestingly, very long fasts can push blood sugar into the low range even in people without diabetes — yet cause no symptoms at all. This is because the body slowly adapts to lower glucose levels over time.
Medications That Lower Blood Sugar
Several medications can cause blood sugar to drop. These include:
Insulin (used in both Type 1 and Type 2 diabetes)
Sulfonylureas — a class of oral diabetes drugs, such as glyburide and glimepiride
Salicylates (including high-dose aspirin)
Certain heart medications, such as disopyramide and beta-blockers
Some antibiotics, such as cotrimoxazole, particularly in people with kidney problems
Haloperidol and quinine in certain circumstances
Because sulfonylureas stay active in your body for a long time, low blood sugar can return up to 48 hours after taking a dose. This means treatment may need to continue well after the first episode is resolved.
Alcohol and Exercise
Drinking alcohol, especially on an empty stomach, can cause blood sugar to fall significantly. Intense or prolonged physical activity can have the same effect. Both can deplete your body’s stored glucose faster than it can be replaced.
Other Medical Conditions
Certain health conditions also raise the risk. These include kidney failure, congestive heart failure, Addison’s disease, septicaemia (a serious blood infection), anorexia nervosa, and rare conditions such as insulinoma — a tumour of the pancreas that produces too much insulin.
Recognising the Symptoms of Low Blood Sugar
Symptoms of low blood sugar tend to fall into two groups. Knowing both groups can help you act quickly.
Autonomic (Early Warning) Symptoms
These are your body’s first alarm signals. They are driven by your nervous system releasing stress hormones in response to falling glucose. Common early symptoms include:
Sweating
Shakiness or trembling
Heart palpitations
Anxiety or nervousness
Hunger
These symptoms typically begin when blood sugar drops to around 60 mg/dL (3.3 mmol/L). Acting at this stage is much safer and easier than waiting for things to get worse.
Neuroglycopenic (Brain-Related) Symptoms
If blood sugar continues to fall below 50 mg/dL (2.8 mmol/L), your brain begins to suffer from lack of glucose. This is called neuroglycopenia. Symptoms at this stage include:
Fatigue and weakness
Dizziness and nausea
Blurred or double vision
Difficulty speaking
Tingling around the mouth (oral paraesthesia)
Confusion or unusual behaviour
Drowsiness
In severe cases, low blood sugar can cause seizures, loss of consciousness, or even cardiac problems in people with pre-existing heart conditions. According to Mayo Clinic’s guide on hypoglycaemia, acting quickly at the first signs is essential to prevent serious harm.
Low Blood Sugar and Diabetes
Low blood sugar is the most common complication of insulin therapy for people living with Type 1 diabetes. In fact, almost everyone with Type 1 diabetes experiences at least one episode per year.
Nighttime Hypoglycaemia
Many people with Type 1 diabetes experience low blood sugar during the night while they are asleep. Because they are unconscious, they may not feel the early warning signs. This is why overnight monitoring and careful insulin dosing are so important. Talk to your family doctor or diabetes specialist about strategies to reduce nighttime risk.
Type 2 Diabetes and Oral Medications
Low blood sugar can also affect people with Type 2 diabetes, particularly those taking insulin or sulfonylurea medications. As noted above, these oral drugs can remain active in the body for up to two days, meaning a single low episode may require ongoing monitoring and treatment.
Long-Term Risks of Repeated Episodes
Repeated low blood sugar episodes are not just uncomfortable — they can cause lasting harm. Severe hypoglycaemia is linked to irreversible brain damage in extreme cases. Furthermore, research shows that even shorter or milder episodes in children can lead to changes in brain activity and reduced cognitive function. Encouragingly, these changes often improve after several years of well-managed diabetes care.
Low blood sugar also causes about 3–4% of deaths in people treated with insulin. Beyond the physical risks, many people develop a fear of low blood sugar episodes, which can affect daily activities such as driving. Health Canada’s information on insulin products provides guidance on safe use and storage for Canadians.
What to Do When Blood Sugar Drops Low
If you or someone nearby shows signs of low blood sugar, acting quickly matters. The standard approach is called the “15-15 Rule”:
Take 15 grams of fast-acting carbohydrates (for example, 4 glucose tablets, 125 mL of fruit juice, or 3 teaspoons of sugar dissolved in water)
Wait 15 minutes, then check your blood sugar again
If it is still low, repeat the steps
Once blood sugar is back to normal, eat a small snack if your next meal is more than an hour away
If the person is unconscious or cannot swallow safely, do not give anything by mouth. Call 911 immediately. Emergency services can administer glucagon or intravenous glucose. Many Canadians with diabetes are prescribed a glucagon emergency kit — make sure family members and close friends know where it is kept and how to use it.
For more detailed guidance on managing blood sugar emergencies, Healthline’s overview of hypoglycaemia is a helpful resource.
When to See a Doctor
If you experience repeated episodes of low blood sugar, you should speak with your family doctor as soon as possible. Do not wait for your next scheduled appointment — many walk-in clinics across Canada can help with urgent concerns in the meantime.
You should seek immediate medical attention if low blood sugar causes confusion, loss of consciousness, seizures, or chest pain. Call 911 or go to your nearest emergency department right away.
Your doctor may refer you to an endocrinologist — a specialist in hormones and metabolism — if the cause of your low blood sugar is not clear, or if episodes are frequent and hard to control. Provincial health plans across Canada generally cover these specialist referrals when arranged through your family doctor.
It is also worth keeping a log of your symptoms, the time they occurred, and what you had eaten beforehand. This information helps your healthcare provider identify patterns and adjust your treatment plan. Always consult your doctor before making any changes to your diabetes medication or diet.
Frequently Asked Questions About Low Blood Sugar
What are the first signs of low blood sugar?
The first signs of low blood sugar are usually sweating, shakiness, heart palpitations, and anxiety. These early warning symptoms are triggered by your nervous system and typically appear when blood glucose falls to around 60 mg/dL. Acting quickly at this stage — by eating fast-acting carbohydrates — can prevent the episode from getting worse.
What causes low blood sugar in people without diabetes?
Low blood sugar can occur in people without diabetes due to prolonged fasting, heavy alcohol use, intense exercise, or certain medications like high-dose aspirin. Rare causes include insulinoma (a pancreatic tumour) and some serious illnesses such as kidney or liver failure. If you experience unexplained low blood sugar episodes, your family doctor can help identify the underlying cause.
How do I quickly raise low blood sugar?
The fastest way to raise low blood sugar is to take 15 grams of fast-acting carbohydrates, such as glucose tablets, fruit juice, or regular soda (not diet). Wait 15 minutes and recheck your blood sugar, repeating if needed. Always follow up with a small snack if your next meal is still more than an hour away.
Can low blood sugar cause long-term brain damage?
Severe and repeated episodes of low blood sugar can cause lasting changes in brain function, particularly in children. Research has shown links between recurrent hypoglycaemia and reduced cognitive performance, though these effects can improve with years of well-managed diabetes care. This is one important reason why keeping blood sugar stable is a key goal of diabetes treatment.
Is low blood sugar dangerous while driving?
Yes, low blood sugar is a serious safety concern while driving, as it can cause confusion, blurred vision, and slowed reaction time. Canadian road safety guidelines recommend checking your blood sugar before getting behind the wheel and keeping fast-acting carbohydrates in your vehicle. If you feel symptoms of low blood sugar while driving, pull over safely and treat it immediately before continuing.
What blood sugar level is considered dangerously low?
A blood sugar level below 40 mg/dL (2.2 mmol/L) is considered severely low and requires immediate treatment. At this level, brain function is seriously impaired and the risk of seizures or loss of consciousness rises sharply. If someone cannot swallow safely or loses consciousness due to low blood sugar, call 911 right away.
Key Takeaways
Low blood sugar occurs when blood glucose falls below the normal range, typically under 65 mg/dL (3.6 mmol/L).
Early symptoms include sweating, shakiness, palpitations, and hunger. Later symptoms affect the brain and can include confusion and loss of consciousness.
It is most common in people with Type 1 diabetes using insulin, but it can also affect those with Type 2 diabetes or people without diabetes at all.
The 15-15 Rule — 15 grams of fast-acting carbohydrates, wait 15 minutes, recheck — is the standard first response for a mild to moderate episode.
Repeated or severe episodes can cause lasting harm, including brain and heart complications.
If episodes keep happening, speak with your family doctor or visit a walk-in clinic. Do not adjust your diabetes medications on your own.
Always consult a healthcare professional before making changes to your diet, medications, or diabetes management plan.
