Quick answer: here is how GLP-1 drugs work. They copy glucagon-like peptide-1 (GLP-1), a hormone your gut releases after meals. They boost insulin and lower glucagon only when blood sugar is high, slow stomach emptying, and act on brain areas that control hunger, so people eat less.[1] In trials, that led to average weight loss of 14.9% with semaglutide 2.4 mg over 68 weeks and up to 20.9% with tirzepatide 15 mg over 72 weeks.[7][8]
This guide explains the biology in plain terms. For drug-by-drug details, see our profiles of semaglutide, tirzepatide, liraglutide and dulaglutide, and the GLP-1 basics hub.
What GLP-1 is: your body’s own “incretin”
GLP-1 is a 30-amino-acid hormone made by L-cells in the lining of the lower gut.[3] Along with GIP, made by K-cells in the upper gut, it is one of two incretin hormones: gut signals released when you eat that tell the pancreas to release insulin.[2]
Incretins explain a curious fact. Glucose taken by mouth triggers two to three times more insulin than the same glucose given into a vein, because swallowing food switches on the gut hormones.[2] In a classic 1986 study, incretins accounted for about 73% of the insulin response to oral glucose in healthy people but only 36% in people with type 2 diabetes.[4] Restoring that lost signal is where GLP-1 drugs began.
How GLP-1 drugs work, step by step
- You eat, and the drug is already waiting. Unlike natural GLP-1, which rises briefly after meals, a weekly injection keeps GLP-1 receptors activated around the clock.
- The pancreas releases more insulin, but only when needed. GLP-1 drugs increase insulin and reduce glucagon in a glucose-dependent way, so the effect fades as blood sugar returns to normal.[5] That’s why low blood sugar is uncommon unless they’re combined with insulin or a sulfonylurea.
- The stomach empties more slowly. Food stays in the stomach longer, which blunts blood sugar spikes and adds to fullness.[1] For tirzepatide this effect is largest after the first dose and fades over time.[6]
- The brain gets a stronger “full” signal. GLP-1 receptors sit in several brain areas that regulate appetite, and the Wegovy label reports that, in animals, semaglutide reached and activated neurons in those regions.[5]
- You eat less, and weight falls. The Wegovy label says semaglutide lowers calorie intake, likely through effects on appetite, and causes more fat loss than lean mass loss.[5]
Why natural GLP-1 can’t be used as a drug
Natural GLP-1 is broken down within minutes by an enzyme called DPP-4 and cleared by the kidneys.[1][3] Drug makers solved this in two ways:
- Make GLP-1 resistant to breakdown. Semaglutide shares 94% of its sequence with human GLP-1 but has changes that protect it from DPP-4.[5]
- Hitch a ride on albumin. Semaglutide carries a C18 fatty acid chain and tirzepatide a C20 chain. Both bind albumin, the most common blood protein, which slows their clearance.[5][6]
The result: semaglutide’s half-life is about 1 week and tirzepatide’s is about 5 to 6 days, which allows once-weekly injections.[5][6] The flip side is that semaglutide stays in the blood for 5 to 7 weeks after the last dose.[5] Our guides on how long semaglutide stays in your system and tirzepatide explain the timing.
A different class, DPP-4 inhibitor pills, blocks the enzyme so your own GLP-1 lasts longer. They lower blood sugar but are generally considered weight-neutral, unlike GLP-1 receptor agonists.
Single, dual and triple agonists
| Type | What it activates | Examples |
|---|---|---|
| GLP-1 receptor agonist | GLP-1 receptor | Semaglutide, liraglutide, dulaglutide, exenatide |
| Oral non-peptide GLP-1 agonist | GLP-1 receptor (small molecule pill) | Orforglipron |
| Dual agonist | GLP-1 + GIP receptors | Tirzepatide |
| Dual agonist (investigational) | GLP-1 + glucagon receptors | Survodutide, mazdutide |
| Triple agonist (investigational) | GLP-1 + GIP + glucagon receptors | Retatrutide |
Tirzepatide binds both incretin receptors.[6] In separate trials, it produced more weight loss than semaglutide did against placebo, and our semaglutide vs tirzepatide comparison covers the head-to-head data. Adding glucagon receptor activity is being tested to raise energy use and reduce liver fat. For a full lineup, see GLP-1 drugs compared.
Injections vs pills
Because GLP-1 drugs are peptides, stomach acid and digestive enzymes break most of them down if swallowed. Two approaches get around this. The first is an oral semaglutide tablet formulated with an absorption enhancer (salcaprozate sodium). Wegovy is now available as a daily tablet, with a 25 mg maintenance dose for weight reduction, and it must be taken with care: you wait at least 30 minutes before eating, drinking or taking other pills.[5] Our comparisons of oral vs injectable semaglutide and Rybelsus vs Ozempic cover the details.
The second approach skips peptides altogether. Orforglipron is a small-molecule pill that activates the same GLP-1 receptor without being a peptide, so it doesn’t need the same fasting rules. See orforglipron vs tirzepatide for how it compares. Whatever the form, the receptor being switched on is the same, which is why benefits and side effects follow a similar pattern.
Why side effects happen
The same actions that drive weight loss also explain the common side effects. Slower stomach emptying and signals to brain areas involved in nausea help explain why nausea, vomiting and constipation are the most frequent complaints, especially after dose increases.[1] Our GLP-1 side effects hub lists the rates for each drug.
What happens when you stop
GLP-1 drugs manage appetite only while they’re active. Once the drug clears, hunger signals return. In the STEP 1 extension, people regained about two-thirds of their lost weight within a year of stopping semaglutide.[9] See how to keep weight off after a GLP-1 and stopping GLP-1 medication.
Your prescriber can explain which drug fits your health history. These medicines are prescription-only, and the dose is set by a clinician.
