A common worry when starting a GLP-1 is whether it will send your blood sugar crashing. On its own, it usually won't; the way these medications work makes dangerous lows unlikely. That changes once a GLP-1 is added on top of insulin or a sulfonylurea, and that combination is where the risk is real enough to plan around.
Why a GLP-1 rarely causes lows on its own
GLP-1 receptor agonists prompt the pancreas to release insulin in a glucose-dependent way: they mainly stimulate insulin secretion when blood sugar is elevated, and the effect tapers off as glucose returns toward normal. Because the drug largely stops driving insulin release once you're no longer high, it doesn't tend to push blood sugar into the danger zone by itself. That's why reviews of GLP-1 safety note these drugs don't cause hypoglycemia when combined with medications like metformin, which also carry low intrinsic risk (Filippatos et al., Rev Diabet Stud, 2015, DOI (external link)).
Low risk on its own doesn't mean low risk in every combination.
Which combinations raise the risk
Two classes of diabetes medication work differently from a GLP-1: they lower blood sugar whether or not it is currently high.
- Insulin directly lowers glucose by the amount you dose, independent of your current level.
- Sulfonylureas (glipizide, glimepiride, glyburide) and the related meglitinides push the pancreas to release insulin continuously, not just when blood sugar is elevated.
Add a GLP-1 on top of either, and the glucose-lowering effects add together. That's where lows happen. The same review notes that when a GLP-1 is combined with a sulfonylurea or insulin, the dose of that companion drug may need to come down to reduce the risk of hypoglycemic episodes (Filippatos et al., Rev Diabet Stud, 2015, DOI (external link)). An umbrella review of adding a GLP-1 to insulin-based regimens found the combination significantly increased the risk of hypoglycemia compared with placebo (Chai et al., J Diabetes Res, 2024, DOI (external link)). And in a large trial, hypoglycemia was notably concentrated among participants also taking a sulfonylurea (Del Prato et al., Lancet, 2021, DOI (external link)).
The prescribing information says so directly. The FDA label for semaglutide (Ozempic) states: "When initiating OZEMPIC, consider reducing the dose of concomitantly administered insulin secretagogue (such as sulfonylureas) or insulin to reduce the risk of hypoglycemia." The American Diabetes Association's Standards of Care in Diabetes—2026 (recommendation 9.17) likewise advises that when starting a new glucose-lowering medication, clinicians reassess the need for and dose of drugs with higher hypoglycemia risk: sulfonylureas, meglitinides, and insulin (Diabetes Care, 2026 (external link)). So a dose change is expected, not a sign something went wrong.
If you are switching between diabetes and weight-management dosing of the same drug, the specifics of your regimen matter here too; see switching between diabetes and weight-management dosing.
Recognizing a low, and the 15-15 rule
Hypoglycemia (blood sugar below 70 mg/dL, per CDC guidance) can come on quickly. Common early signs include shakiness, sweating, a fast or pounding heartbeat, sudden hunger, irritability or confusion, dizziness, and difficulty concentrating.
For a low that you can still treat yourself, the CDC recommends the 15-15 rule: eat or drink 15 grams of fast-acting carbohydrate (for example, glucose tablets or 4 ounces of juice), wait 15 minutes, then recheck your blood sugar. If it is still below 70 mg/dL, repeat the 15 grams and recheck, continuing until you are back in range, then follow with a balanced snack or meal. Fast-acting carbohydrate matters here; a candy bar with fat or protein raises blood sugar more slowly.
When to seek urgent care
Treat these as emergencies, not wait-and-see situations:
- Confusion, slurred speech, or loss of consciousness
- A seizure
- A low that does not come back up after repeating the 15-15 rule
- Any low in someone unable to safely swallow
Severe hypoglycemia can require a glucagon rescue and emergency help, so call your local emergency number. If you and your prescriber have determined you are at risk, ask whether you should carry a glucagon kit and make sure someone close to you knows how to use it. (This is a different emergency pathway than the GI-driven problems covered in our GI side effects overview. And unlike an accidental double dose of the GLP-1 itself, a low from insulin or a sulfonylurea can escalate within minutes.)
What to settle with your prescriber first
The most important point: don't adjust or stop your insulin or sulfonylurea on your own. Dose changes should be planned with the person who prescribes them, ideally before your first GLP-1 injection. Things to raise:
- Whether your insulin or sulfonylurea dose should be lowered now, and by how much
- How often to check your blood sugar in the first weeks, and what number should prompt a call
- Whether you should have a glucagon kit on hand
- What to do on days you eat far less than usual. GLP-1 appetite suppression plus an unchanged insulin or sulfonylurea dose is a classic setup for a low.
If you go into that conversation knowing the combination is the risk, not the GLP-1 alone, you and your prescriber can plan ahead instead of reacting to a scary number later.
This guide is general education, not medical advice. Your diabetes regimen is specific to you; make any changes to insulin, sulfonylureas, or your GLP-1 only in consultation with your prescriber or pharmacist.
Research summarized here was identified in part using PubMed. Please consult the linked sources and your care team before acting on any medical information.