Why Insulin and Certain Diabetes Medicines Cause Low Blood Sugar

Low blood sugar (hypoglycemia—typically a reading below 70 mg/dL) is one of the most concerning experiences for anyone taking diabetes treatment.

Understanding why low blood sugar happens empowers you to prevent crashes without living in fear of your prescribed medication.

Which Diabetes Treatments Can Cause Low Sugar?

• All Types of Injected Insulin: Insulin directly forces glucose out of the blood and into cells. If there isn't enough glucose in your blood to match the insulin dose, blood sugar drops rapidly.
• Sulfonylurea Tablets (Glimepiride/Getryl, Amaryl, Glibenclamide): These tablets signal your pancreas to pump out extra insulin continuously—even if your blood sugar is already dropping.
• Medicines That Rarely Cause Low Sugar: Metformin, Januvia (DPP-4 inhibitors), and Jardiance (SGLT2 inhibitors) rarely cause hypoglycemia when taken alone.

The Four Main Triggers of Low Blood Sugar

1. 1Delayed or Skipped Meals: Taking mealtime insulin or a sulfonylurea tablet and then delaying your meal (e.g., stuck in traffic or long cooking delays) means insulin is active without incoming food carbs.
2. 2Unplanned Physical Activity: Taking a long walk, doing heavy housework, or playing sports increases muscle glucose consumption, making your usual insulin dose work twice as fast.
3. 3Incorrect Dose Measurement: Accidentally misreading syringe markings or dialing the wrong units on an insulin pen.
4. 4Alcohol Consumption: Alcohol stops your liver from releasing its normal backup glucose into the blood, increasing nighttime hypo risk.

The Mechanism: Why Insulin and Sulfonylureas Behave Differently From Other Medicines

The reason insulin and sulfonylureas cause low blood sugar, while Metformin, DPP-4 inhibitors, and SGLT2 inhibitors rarely do, comes down to whether the medicine's glucose-lowering effect responds to how much sugar is actually in your blood. Sulfonylureas like Glimepiride work by closing a specific channel on the pancreas's insulin-producing cells, which forces them to release insulin. According to research published by the American Diabetes Association (opens in a new tab), this effect is not glucose-dependent — the pancreas keeps releasing insulin because the tablet is telling it to, even after blood sugar has already dropped to a normal or low level. Injected insulin has the same underlying problem from a different angle: your body's own natural insulin release constantly adjusts itself in real time as glucose falls, but insulin from a pen or vial keeps working in your bloodstream for its full duration regardless of what your blood sugar happens to be doing in that moment.

Metformin, DPP-4 inhibitors (Januvia, Galvus), and SGLT2 inhibitors (Jardiance, Forxiga) don't share this problem. Metformin doesn't trigger extra insulin release at all — it changes how the liver and muscles handle glucose that's already there. DPP-4 inhibitors only boost insulin release when blood sugar is already elevated after a meal, and taper off as glucose normalizes. That built-in off switch is why these classes carry a very low hypoglycemia risk when used alone, and why doctors reach for them first in people who are more vulnerable to lows.

Preventing Low Blood Sugar Before It Starts

• Keep meal and dose timing consistent day to day — insulin and sulfonylureas are dosed around an expected meal, so an unpredictable eating schedule makes lows harder to avoid.
• Adjust for extra activity in advance where possible: a longer walk, wedding dancing, or a heavy household chore day may call for a smaller dose or an extra snack, per your doctor's guidance, not a guess in the moment.
• Test before driving, exercising, or sleeping if you're on insulin or a sulfonylurea, especially when starting a new dose or medicine.
• Never skip a meal after taking a dose that's designed to cover that meal — if you truly can't eat, tell your doctor how to adjust the dose rather than skipping the meal only.

Responding to a Low Blood Sugar Episode

The ADA (opens in a new tab) recommends the 15-15 rule as the standard first response once a reading is confirmed below 70 mg/dL: eat or drink 15 grams of fast-acting carbohydrate, wait 15 minutes, then retest. Repeat the cycle if the reading is still under 70 mg/dL. Once your blood sugar is back in range, eating a small snack with some protein or fat can help keep it steady, particularly if your next meal is still some time away. A reading below roughly 54 mg/dL, or any low accompanied by confusion, seizure, or loss of consciousness, is severe hypoglycemia — it needs glucagon or emergency medical care rather than the 15-15 rule, since the person may not be able to safely eat or drink.

“A low blood sugar event is not a personal mistake—it is a signal that your food, activity, and dose timing need a small adjustment.”

Track your low sugar episodes

Log any low readings in Diatic along with notes about timing and food to spot patterns before your next clinic visit.

Log a reading (opens Diatic on Google Play in a new tab)