A GLP-1 lowers blood sugar largely by amplifying the insulin response you already produce and damping the hormone that raises glucose. Both effects are glucose-dependent: when your sugar is not high, they largely stand down. That is why these drugs cause remarkably little hypoglycemia by themselves.
⛔ Insulin and sulfonylureas carry no such governor. They push insulin whether or not it is wanted. Layer a GLP-1 on top without touching them, and the danger comes from the combination rather than from the new drug — which is exactly why every one of these labels warns about it, and why something usually has to come down before the GLP-1 goes up.
What to do with each
| Medication | Examples | Typically |
|---|---|---|
| Long-acting insulin | Lantus, Tresiba, Toujeo, Levemir, Basaglar | Comes down, commonly by about a fifth to a quarter at the start, and further if your numbers are already at target. |
| Mealtime insulin | Humalog, NovoLog, Apidra, Lyumjev, Fiasp | Comes down more, often by a quarter to a half, because you are about to eat less. Some people stop it altogether once the GLP-1 reaches a working dose. |
| Sulfonylureas | Glipizide, glimepiride, glyburide | Often stopped, or roughly halved if kept. This is the highest-hypoglycemia oral class, and adding a GLP-1 is a natural moment to reconsider it. |
| Meglitinides | Repaglinide, nateglinide | Same logic as sulfonylureas — halved or stopped. |
| Metformin | Glucophage, Glumetza, Fortamet | Unchanged. Almost no hypoglycemia risk of its own and a natural partner to a GLP-1. |
| SGLT2 inhibitors | Jardiance, Farxiga, Invokana | Unchanged. Different mechanism, no shared hypoglycemia risk, and benefits to heart and kidney that add rather than overlap. |
| DPP-4 inhibitors | Januvia, Tradjenta, Onglyza | Stopped. They work on the same incretin pathway a GLP-1 now occupies far more powerfully, so they add nothing. |
| Pioglitazone | Actos | Unchanged, though fluid retention can disguise weight progress. |
| Another GLP-1 | Victoza, Saxenda, Trulicity, Byetta | One stops before the other starts. Combinations have not been studied and are not recommended. |
⚠ On those percentages. The labels themselves say to consider lowering the insulin or secretagogue dose without naming a figure. The numbers above reflect common clinical practice rather than label instruction, and your prescriber is titrating to your readings rather than to a table. Do not adjust insulin on the strength of a reference card.
Recognizing a hypo
The early signs are physical and easy to dismiss as something else.
- Shaking or trembling hands — usually the first thing.
- Cold sweat or clammy skin with no reason to be sweating.
- Confusion, trouble concentrating, slurred words.
- Dizziness or feeling about to faint.
- A pounding or racing heart.
- Blurred vision, sudden irritability, sudden hunger, headache.
Treat it with roughly 15 to 20 grams of fast sugar — a small glass of juice, a few glucose tablets, a spoon of honey, a handful of hard sweets. Wait a quarter of an hour, test again, and repeat if you are still below 70 mg/dL.
Checking it is working
Standard practice is an A1C every three months until you are at target, then twice a year. After a GLP-1 goes in, most prescribers look sooner — a fasting glucose or a continuous-monitor snapshot at four to eight weeks — specifically to catch over-correction before the formal three-month reading.
That first proper A1C tells you whether the reduction you made at the start was right, insufficient, or too much.
Call your prescriber if
- A severe hypo. Passing out, a seizure, or needing someone else to help you. A single one warrants a same-day call.
- Three or more mild lows inside a week, even ones you handled yourself. The other drug is too high.
- A1C drifting under 6.0% while still on insulin or a sulfonylurea. Looks like success, carries real risk.
- Fasting glucose under 80 mg/dL for a week or more. The basal insulin needs to come down again.
- Readings below 60 mg/dL that you cannot feel at all. Losing hypo awareness means the regimen needs rethinking now.
What this does not cover
Diabetes co-medication only. It does not cover the non-diabetes interactions — how a slowed stomach affects the contraceptive pill or levothyroxine timing, or warfarin monitoring. Nor type 1 diabetes, diabetes in pregnancy, children, or ketoacidosis planning. Sick days and surgery have their own page.
Nothing here is a reason to change an insulin dose on your own. This is a map of the conversation, not a substitute for it.
Where to go next
- All cheat sheets — the full reference library.
- Sick-day guide — holding doses during illness, dehydration and surgery.
- Pregnancy and contraception — the other interaction people miss.
- Brand names against molecules — why the diabetes and weight versions of one drug are priced so differently.