Low Blood Sugar: Symptoms, Causes, and the 15 Gram Rule

You started insulin or a new diabetes pill, and you feel shaky and sweaty before lunch. Or your meter shows a number lower than you have ever seen. Low blood sugar can come on fast, but it can be treated quickly when you know what to do. This guide explains the levels, the signs, and the steps.

What low blood sugar is

Low blood sugar (hypoglycemia) means the amount of glucose in your blood has dropped below a safe level. Your brain runs mainly on glucose, so low levels affect how you think and act.

Low blood sugar is mostly a concern for people with diabetes who take certain medicines. It is uncommon in people without diabetes.

What your doctor looks at

The levels

The American Diabetes Association (ADA) uses three levels [1]:

• Level 1: glucose below 3.9 mmol/L (70 mg/dL) and at or above 3.0 mmol/L (54 mg/dL)
• Level 2: glucose below 3.0 mmol/L (54 mg/dL). This needs action right away.
• Level 3: a severe event where a person is confused or unable to act and needs help from someone else to recover, whatever the number

Diabetes Canada uses a level below 4.0 mmol/L as low for people treated with insulin or insulin-releasing pills [2]. It groups episodes as mild, moderate, or severe, based on symptoms and whether the person can treat themselves [2].

The symptoms

Early symptoms come from stress hormones [2] [4]:

• Shakiness or trembling
• Sweating
• Fast or pounding heartbeat
• Hunger
• Anxiety or irritability
• Tingling around the mouth

As glucose falls further, the brain is affected [2] [4]:

• Trouble concentrating or confusion
• Weakness or drowsiness
• Blurred vision
• Trouble speaking
• Unusual behaviour
• Seizures or passing out

Lows can also happen during sleep. Signs include nightmares, sweating through sheets, or a headache on waking.

The causes

Most lows in diabetes come from medicines that raise insulin levels [1] [2]:

• Insulin
• Sulfonylurea pills, such as gliclazide, glyburide, or glimepiride
• Meglitinides, such as repaglinide

Lows become more likely when you:

• Skip or delay a meal or eat fewer carbohydrates than usual
• Are more active than usual
• Drink alcohol, especially without food
• Take a dose at the wrong time or take too much
• Have kidney disease, which can make some medicines last longer
• Lose weight, which can lower the dose you need

Metformin, SGLT2 inhibitors, GLP-1 drugs, and DPP-4 inhibitors rarely cause lows on their own [1].

The 15 gram rule

If you can swallow safely and your glucose is low, follow these steps [2] [3]:

• Eat or drink 15 grams of fast-acting sugar.
• Wait 15 minutes, then check your glucose again.
• If it is still low, take another 15 grams.
• Once your glucose is back in range, eat your next meal or a snack with protein and carbohydrate if your meal is more than an hour away.

What 15 grams looks like

• Glucose tablets: check the label. Often 3 to 4 tablets.
• Juice or regular, non-diet soft drink: the CDC lists half a cup (about 125 mL) [3]. Diabetes Canada lists three quarters of a cup (175 mL) [2]. Check the sugar on the label.
• 1 tablespoon (15 mL) of sugar, honey, or syrup [3].
• Hard candies or jelly beans: check the label for how many.

Avoid chocolate, cookies, and other foods high in fat or fibre as your first treatment. Fat and fibre slow how fast sugar gets into your blood [3].

For severe lows in a person who is awake and can swallow, Diabetes Canada suggests 20 grams [2].

Glucagon and severe lows

Glucagon is a hormone that tells the liver to release stored sugar. It comes as an injection or a nasal spray. The ADA recommends glucagon for anyone at risk of level 2 or level 3 lows [1].

If a person is confused and cannot swallow safely, or has passed out:

• Do not put food or drink in their mouth.
• Give glucagon if it is available and you know how.
• Call 911.
• Turn the person on their side.

After glucagon, the person usually wakes within about 15 minutes [3]. Once awake and able to swallow, they should have sugar and then a snack.

Preventing lows

• Check your glucose more often when you change medicines, exercise, drink alcohol, or are sick.
• Carry fast sugar with you at all times.
• Wear medical ID.
• Teach family, friends, and coworkers the signs and how to help.
• Check before you drive, and do not drive if your glucose is low. Ask your clinician about the rules where you live.
• A continuous glucose monitor with alarms can help some people catch lows early.

Questions to ask your clinician

• Which of my medicines can cause low blood sugar?
• What glucose level should make me treat a low?
• Should I have glucagon, and who should learn to use it?
• Should my doses change on days I exercise or drink alcohol?
• What should I do with my medicines on sick days?
• Would a continuous glucose monitor help me?
• What are the driving rules for me?

When to get care quickly

Call 911 if a person with low blood sugar:

• Is confused and cannot swallow safely
• Has a seizure or passes out
• Does not improve after glucagon or repeated treatment

Contact your clinician soon if you have repeated lows, lows at night, or lows without warning symptoms.

Bottom line

Low blood sugar is common in people who take insulin or sulfonylureas, and it is treatable. Learn the signs, use the 15 gram rule, and recheck in 15 minutes. Keep glucagon on hand if you are at risk of severe lows, and make sure the people around you know how to help.

Primary AI can show you the guideline and study sources behind this topic in plain language, so you can bring better questions to your appointment.