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. But that picture changes as soon as a GLP-1 is layered on top of insulin or a sulfonylurea, and that combination is exactly where the risk becomes 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 — meaning they mainly stimulate insulin secretion when blood sugar is elevated, and that effect tapers off as glucose returns toward normal. Because the drug largely stops driving insulin release once you are no longer high, it does not tend to push blood sugar down into the danger zone by itself. This is why reviews of GLP-1 safety note that these drugs do not cause hypoglycemia when combined with medications like metformin, which also carry low intrinsic risk (Filippatos et al., Rev Diabet Stud, 2015, DOI (external link)).
The important word is intrinsic. Low risk on its own is not the same as 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.
Stack a GLP-1 on top of either, and the glucose-lowering effects add together — which is 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 have to be decreased to reduce the risk of hypoglycemic episodes (Filippatos et al., Rev Diabet Stud, 2015, ). 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., , 2024, ). And in a large trial, hypoglycemia was notably concentrated among participants also taking a sulfonylurea (Del Prato et al., , 2021, ).