Last updated: June 2026 · Reviewed June 2026 · Built by the InjectBuddy team
GLP-1 Terms Glossary and the maths behind each word
A GLP-1 terms glossary defines the words on a semaglutide or tirzepatide label — receptor agonist, incretin, concentration, titration, U-100 units — and shows the arithmetic each one stands for. This guide walks through each term in a plain-language table, works them through seven real syringe-draw examples, and answers the questions people ask most.
Key takeaways:
- Dose (mg) is what you are prescribed; volume (mL) is what you draw. Concentration (mg/mL) is the bridge between them.
- U-100 units are volume marks: 1 mL = 100 units, so units = mL × 100.
- Titration is the planned weekly step-up that the label builds in to ease side effects.
- Know the dose and concentration? Confirm the draw in the semaglutide dose calculator.
The vocabulary that actually changes your draw
Most GLP-1 confusion is not about pharmacology — it is three words pretending to be one. A dose is the mass of active drug your prescriber sets, written in milligrams (mg). Volume is the amount of liquid that mass lives in, written in milliliters (mL). Concentration (mg/mL) is the exchange rate between them. Change the concentration and the same dose needs a different volume, exactly like the same amount of cordial making a different number of glasses depending on how much water you add.
GLP-1 receptor agonists are incretin-based medicines: they mimic a gut hormone that signals fullness and helps regulate glucose. Once-weekly semaglutide 2.4 mg produced clinically meaningful weight reduction in the STEP 1 trial (Wilding et al., NEJM 2021). Tirzepatide adds a second receptor — it is a dual GIP and GLP-1 agonist — and SURMOUNT-1 reported substantial weight reduction across its dose tiers (Jastreboff et al., NEJM 2022). None of that pharmacology decides your draw; the dose and concentration do.
The definition table
This is the glossary in one place: each term, a plain definition, and a concrete example of the number it produces.
| Term | Plain definition | Example |
|---|---|---|
| GLP-1 receptor agonist | A drug that copies the GLP-1 gut hormone | Semaglutide (Wegovy, Ozempic) |
| Dual GIP/GLP-1 agonist | Acts on two incretin receptors at once | Tirzepatide (Mounjaro, Zepbound) |
| Dose | Mass of drug prescribed, in mg | 0.25 mg starting dose |
| Concentration | Drug per mL of liquid, in mg/mL | 2.4 mg in 0.75 mL = 3.2 mg/mL |
| Volume | Liquid amount drawn, in mL | 0.25 mg ÷ 1 mg/mL = 0.25 mL |
| U-100 unit | Volume mark where 100 units = 1 mL | 0.25 mL = 25 units |
| Titration | Planned weekly dose step-up | 0.25 → 0.5 → 1.0 mg |
| Maintenance dose | Target dose held after titration | Semaglutide 2.4 mg weekly |
| mcg | Microgram; 1 mg = 1000 mcg | 0.25 mg = 250 mcg |
| Reconstitution | Adding water to a powder vial | 5 mg + 2 mL = 2.5 mg/mL |
How this is calculated
Every GLP-1 draw is the same two-step arithmetic, no matter the brand:
- Volume (mL) = dose (mg) ÷ concentration (mg/mL). Keep both numbers in the same mass unit — mg with mg/mL, or mcg with mcg/mL.
- Units = volume (mL) × 100 on a U-100 insulin syringe, because that syringe prints 100 marks per mL.
That is the whole engine. A prefilled pen does this for you and hides the volume; a vial-and-syringe setup makes you do it. Where a vial is a powder, you first set the concentration yourself by reconstituting — adding a measured volume of water — before either step runs. Below, the same two lines of maths play out across seven real situations. None of this is medical advice; it is the measurement, and the semaglutide dose calculator reproduces it instantly.
The terms in action: seven worked examples
Concentration from a compounded vial
A 5 mg semaglutide vial reconstituted with 2 mL of bacteriostatic water holds 5 ÷ 2 = 2.5 mg/mL. That single number now drives every draw from this vial.
Starting dose → volume → units
At 2.5 mg/mL, a 0.25 mg starting dose is 0.25 ÷ 2.5 = 0.1 mL. On a U-100 syringe that is 0.1 × 100 = 10 units.
Titration step changes the units
Same 2.5 mg/mL vial, week-five dose of 0.5 mg: 0.5 ÷ 2.5 = 0.2 mL = 20 units. The dose doubled, so the draw doubled — titration in plain arithmetic.
mg vs mcg trap
A label reading "250 mcg" is the same as 0.25 mg, because 1 mg = 1000 mcg. Read it as 250 mg and you would draw a 1000× overdose — always convert to one unit first.
Why water volume matters
Reconstitute the same 5 mg vial with 1 mL instead of 2 mL and concentration becomes 5 mg/mL. Now 0.25 mg is 0.05 mL = 5 units — half the marks for the identical dose.
Tirzepatide maintenance draw
A 10 mg/mL tirzepatide vial, prescribed 5 mg: 5 ÷ 10 = 0.5 mL = 50 units, which fills half of a 1 mL U-100 syringe.
Reading a prefilled pen back to mg/mL
A Wegovy 2.4 mg pen delivers its dose in 0.75 mL, so its concentration is 2.4 ÷ 0.75 = 3.2 mg/mL — useful for sanity-checking a vial against the branded product.
Maintenance dose on a smaller syringe
At 3.2 mg/mL, a 2.4 mg maintenance dose is 2.4 ÷ 3.2 = 0.75 mL. On a 1 mL U-100 syringe that is 75 units — three-quarters of the barrel for a single weekly shot.
Common glossary mix-ups
The most expensive mistake is borrowing another person's syringe-unit number without matching concentration. "Draw 20 units" only means a fixed dose if your mg/mL is identical to theirs; on a stronger vial those same 20 units carry more drug. The fix is to never copy units — copy the dose in mg and run your own two-line maths.
The second is sliding between mg and mcg, as the fourth example shows. The third is forgetting that a powder vial has no concentration until you add water, so two people reconstituting the same 5 mg vial differently will read different units for the identical prescribed dose. For the full unit picture, see mg vs mcg vs mL vs units.
Finally, correct maths cannot rescue poor handling: use a fresh sterile needle, follow the label's storage and discard rules, and never use a vial that is cloudy, leaking, expired, or mislabeled.
So, what are the key GLP-1 terms to know?
The terms that change your draw are dose (mg), concentration (mg/mL), and volume (mL). Concentration is the bridge: volume equals dose divided by concentration, then multiplied by 100 to read U-100 syringe units. Titration is the planned weekly step-up built into the label, and reconstitution is setting that concentration yourself by adding a measured volume of water to a powder vial. Run your own numbers through the semaglutide dose calculator to turn any dose and vial strength into an exact draw in seconds.
Frequently asked questions
What are the key GLP-1 terms to know?
What does mg/mL mean on a GLP-1 vial?
How do I turn a mg dose into U-100 syringe units?
Why do GLP-1 doses start low and increase?
Is this glossary a dosing recommendation?
Sources
- Wilding JPH, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity. New England Journal of Medicine, 2021. PubMed PMID 33567185.
- Jastreboff AM, et al. Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1). New England Journal of Medicine, 2022. PubMed PMID 35658024.
- Novo Nordisk. WEGOVY (semaglutide) injection — prescribing information and dose strengths. DailyMed (FDA label). DailyMed: WEGOVY.
- Eli Lilly. MOUNJARO (tirzepatide) injection — prescribing information and dose strengths. DailyMed (FDA label). DailyMed: MOUNJARO.
This guide is for general educational purposes only and does not constitute medical advice. GLP-1 doses, titration schedules, and routes must come from your prescriber and the product label. Always verify your numbers before injecting.