Last updated: June 2026 · Reviewed June 2026 · Built by the InjectBuddy team
What are GLP-1 medications? The four classes, compared
GLP-1 medications are injectable drugs that copy a gut hormone (glucagon-like peptide-1) to lower appetite and blood glucose, and they are now used for type 2 diabetes and weight management. They are not one drug: the term covers a family that ranges from single GLP-1 agonists like semaglutide and liraglutide to dual and triple agonists like tirzepatide and retatrutide, which differ in receptor targets, dosing form, and how often you inject. This guide explains what GLP-1 medications are, compares the four main classes, works through dose-to-units examples for each, and answers the questions people ask most.
- "GLP-1 medication" is a class, not a single product — four head molecules dominate it.
- Single GLP-1 agonists: liraglutide (daily) and semaglutide (weekly).
- Dual GIP/GLP-1 agonist: tirzepatide (weekly). Triple GIP/GLP-1/glucagon agonist: retatrutide (weekly, investigational).
- The class is a prescribing choice; the only thing you calculate at home is dose → syringe units. Check yours in the semaglutide dose calculator.
What "GLP-1 medication" actually means
GLP-1 is an incretin — a hormone the gut releases after eating that nudges the pancreas to release insulin, slows stomach emptying, and signals fullness to the brain. GLP-1 receptor agonists are lab-made molecules that bind the same receptor and hold that signal far longer than the natural hormone, which lasts only minutes. A StatPearls review describes how these incretin mimetics stimulate glucose-dependent insulin secretion, delay gastric emptying, and promote satiety through the hypothalamus.
The newer members do more than copy GLP-1. Tirzepatide also activates the GIP receptor (a second incretin), so it is a dual agonist, sometimes called a twincretin. Retatrutide adds a third target, the glucagon receptor, making it a triple agonist that is still investigational — human dosing outside trials is not established, so treat any retatrutide figure here as illustrative arithmetic, not a protocol. Liraglutide and semaglutide remain single GLP-1 agonists; their main practical difference is half-life, which is why one is daily and one is weekly.
The four classes at a glance
The table below compares the GLP-1 and dual/triple-agonist molecules you are most likely to meet — by receptor mechanism, the form it comes in, and how often it is injected. "Weekly" is possible because the molecule is engineered to resist breakdown, stretching its half-life from minutes to days.
| Molecule | Class / mechanism | Common form | Frequency |
|---|---|---|---|
| Liraglutide | Single GLP-1 agonist | Multi-dose pen | Once daily |
| Semaglutide | Single GLP-1 agonist | Pen or vial | Once weekly |
| Tirzepatide | Dual GIP + GLP-1 agonist | Pen or vial | Once weekly |
| Retatrutide | Triple GIP + GLP-1 + glucagon (investigational) | Vial (trials) | Once weekly |
Notice that mechanism and frequency do not move together: liraglutide and semaglutide share the same single-receptor mechanism yet differ in frequency, while semaglutide and tirzepatide share a weekly schedule despite targeting a different number of receptors. That is why you cannot infer a dose or schedule from the class name alone — you read it off the prescription and label.
How this is calculated
InjectBuddy never picks the drug or the dose — those come from a prescriber. What the calculators do is pure arithmetic: turn a prescribed dose in milligrams into a volume in milliliters, then into the unit mark on a U-100 syringe. Two formulas do all the work:
Volume (mL) = dose (mg) ÷ concentration (mg/mL), then units = volume (mL) × 100 on a U-100 barrel, where 100 units equals 1 mL.
The dose comes from the class-specific titration schedule; the concentration comes from the pen or, for a reconstituted vial, from how much bacteriostatic water you added. Keep mass units consistent — mix mg with mg/mL, or convert first (1 mg = 1,000 mcg). The worked examples below run this for each class so you can see how the same maths covers very different products.
Worked examples by class
Each block converts a representative prescribed dose into a syringe draw. These show the arithmetic only — they are not recommendations, and the retatrutide example is illustrative because no consumer dosing is approved.
Saxenda-style pen delivers 3 mg in 0.5 mL, so concentration = 6 mg/mL. A 1.8 mg step → 1.8 ÷ 6 = 0.3 mL → 30 units on a U-100 syringe. Injected once daily.
Vial reconstituted to 1 mg/mL. Starter dose 0.25 mg → 0.25 ÷ 1 = 0.25 mL → 25 units. Same molecule as liraglutide's class, but dosed weekly.
Same 1 mg/mL vial, maintenance dose 1 mg → 1 ÷ 1 = 1.0 mL → 100 units (a full U-100 barrel). Quadruple the starter volume for quadruple the dose.
10 mg vial + 1 mL bacteriostatic water = 10 mg/mL. A 5 mg dose → 5 ÷ 10 = 0.5 mL → 50 units. Two receptors, but the volume maths is identical.
Same 10 mg vial + 2 mL water = 5 mg/mL. The same 5 mg dose now needs 5 ÷ 5 = 1.0 mL → 100 units. More water doubled the draw for an unchanged dose.
If a 2 mg trial dose came from a vial reconstituted to 4 mg/mL: 2 ÷ 4 = 0.5 mL → 50 units. Triple agonist, but no approved home protocol exists — figures are arithmetic, not advice.
0.25 mg of semaglutide = 250 mcg (×1,000). If a label lists 250 mcg and your calculator expects mg, enter 0.25 — never 250 — or the volume inflates 1,000-fold.
Common mistakes when comparing classes
The biggest error is assuming the class name fixes the dose. Tirzepatide hitting two receptors does not mean you draw "twice" anything — its dose ladder is its own, and you read it off the prescription, not off semaglutide's chart. Copying another person's unit mark is just as risky: the same 50 units is 5 mg at 10 mg/mL but only 2.5 mg at 5 mg/mL, depending on how the vial was mixed.
A second trap is mixing daily and weekly mentally. Liraglutide's daily rhythm means a missed day is a small fraction of weekly exposure, whereas a missed weekly semaglutide or tirzepatide dose is the whole week — handled differently. If you are unsure how an escalation runs, see what a titration schedule is before changing anything. Whatever the class, confirm the final number in the semaglutide dose calculator and follow your prescriber and the product label for storage and handling.
So, what are GLP-1 medications?
GLP-1 medications are a family of injectable drugs that mimic the gut hormone glucagon-like peptide-1, lowering appetite and blood glucose by activating one, two, or three incretin receptors depending on the molecule. The class includes single agonists (liraglutide, semaglutide), a dual GIP/GLP-1 agonist (tirzepatide), and the investigational triple agonist retatrutide. The dose your prescriber sets converts to syringe units using one formula: volume (mL) = dose (mg) divided by concentration (mg/mL), then multiplied by 100 for a U-100 barrel. Run your own numbers in the semaglutide dose calculator.
FAQs
What are GLP-1 medications?
Is a GLP-1 medication the same as semaglutide?
What is the difference between semaglutide and tirzepatide?
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Is this page medical advice?
Sources
- Collins L, Costello RA. Glucagon-Like Peptide-1 Receptor Agonists. StatPearls, NCBI Bookshelf. NBK551568.
- Wilding JPH, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1). N Engl J Med 2021. PMID 33567185.
- Jastreboff AM, et al. Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1). N Engl J Med 2022. PMID 35658024.
- Jastreboff AM, et al. Triple-Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial. N Engl J Med 2023. PMID 37366315.
- Pi-Sunyer X, et al. A Randomized, Controlled Trial of 3.0 mg of Liraglutide in Weight Management (SCALE). N Engl J Med 2015. PMID 26132939.
This guide is for general educational purposes only and does not constitute medical advice. GLP-1 and dual/triple-agonist dosing must come from your prescriber; retatrutide is investigational with no established consumer dosing. Always follow your prescriber's instructions and the product label.