Sulfonylureas are an older, inexpensive class of oral type 2 diabetes medications — glipizide, glimepiride, and glyburide are the common ones. They work by pushing the pancreas to release insulin whether or not blood sugar is actually high, which is effective at lowering A1c but is also why they can cause hypoglycemia on their own.
That mechanism matters for anyone starting a GLP-1. Because a GLP-1 improves blood sugar through its own routes, adding it on top of a sulfonylurea can drive glucose too low; the prescribing information for GLP-1 medications specifically flags this combination, and prescribers commonly reduce or stop the sulfonylurea when a GLP-1 is started. See GLP-1s with insulin or sulfonylureas: understanding hypoglycemia risk.