GLP-1 Basics

Last updated: June 2026 · Reviewed June 2026 · Built by the InjectBuddy team

What Is Semaglutide? A Plain-English Guide

Semaglutide is a once-weekly injectable medicine that copies a natural gut hormone called GLP-1, telling the brain you are full and slowing how fast your stomach empties. That single effect is why it lowers appetite and steadies blood sugar, and why prescribers start the dose low and raise it gradually rather than jumping straight to the full amount.

Key takeaways

Semaglutide mimics GLP-1, a hormone your gut releases after eating, so you feel satisfied with less food.
It is given once a week because its half-life is about 7 days, so one injection lasts the whole week.
Dosing climbs slowly: 0.25 mg to start, stepping up roughly every 4 weeks toward a maintenance dose.
The mg dose and the syringe units are two different things — strength decides how many units one dose is.
Know your prescribed dose and the vial concentration, then check the maths with the semaglutide calculator.

How semaglutide actually works

When you eat, your gut releases a hormone called GLP-1 (glucagon-like peptide-1) that nudges the pancreas to release insulin, slows the stomach, and signals fullness to the brain. The catch is that natural GLP-1 is broken down within minutes. Semaglutide is a re-engineered, long-acting copy: it binds to the same GLP-1 receptors but resists that breakdown, so a single weekly injection keeps the "I'm full" signal switched on for days (StatPearls, 2024). Think of it as a thermostat for appetite — it does not force you to eat less, it lowers the setpoint so smaller meals feel satisfying.

Because the appetite and glucose effects flow from one mechanism, the same molecule treats two different problems under two brand names: Ozempic (for type 2 diabetes) and Wegovy (for weight management). Same drug, different licensed dose ladder and pen sizes. In the STEP 1 trial, weekly semaglutide 2.4 mg produced a mean body-weight reduction of about 14.9% over 68 weeks, versus 2.4% on placebo (Wilding et al., NEJM 2021).

Why the dose starts low and climbs

The most confusing thing for beginners is that the first dose is deliberately too small to do much. That is on purpose. The gut needs time to adapt to the slowed stomach emptying, and starting low keeps side effects — mostly nausea — mild. Every prescribing label uses the same staircase idea: begin at 0.25 mg once weekly, then step up roughly every four weeks (DailyMed, Wegovy label). The 0.25 mg start is a tolerance step, not a treatment dose.

This is why people say "the dose goes up over time" — it is built into how the drug is meant to be used, not a sign anything is wrong. The diagram below shows the typical Wegovy escalation as a staircase.

Typical Wegovy escalation: each step holds for ~4 weeks before the next increase, ending at a 2.4 mg weekly maintenance dose.

Ozempic vs Wegovy: same drug, different ladder

People are often surprised that two products contain the same semaglutide. The difference is the licensed use and the dose steps. This table lays the two brand ladders side by side so the naming stops being mysterious.

Feature	Ozempic	Wegovy
Active drug	Semaglutide	Semaglutide
Licensed for	Type 2 diabetes	Weight management
Start dose	0.25 mg/week	0.25 mg/week
Typical top dose	1-2 mg/week	2.4 mg/week
Frequency	Once weekly	Once weekly
Form	Multi-dose pen	Single-dose pen / syringe

Both are confirmed by their FDA labels: Ozempic pens hold several weekly doses (DailyMed, Ozempic label), while Wegovy is supplied as single-dose 0.25-2.4 mg units (DailyMed, Wegovy label). The escalation idea is identical; only the destination dose differs.

How this is calculated: mg to syringe units

If you use a brand pen, you never do maths — you dial the dose. The arithmetic only appears when you have a vial and a U-100 insulin syringe and need to turn a mg dose into a unit mark. There are just two steps:

Volume (mL) = dose (mg) ÷ concentration (mg/mL). Concentration is how much drug sits in each mL.
Units = volume (mL) × 100. On a U-100 syringe, 1 mL is 100 units, so the unit mark is just the mL shifted two decimal places.

The crucial idea is that the same mg dose is a different number of units at a different concentration. A unit is a measure of liquid, not of drug. The worked examples below all use this exact two-step method; you can reproduce any of them in the semaglutide calculator.

Example 1 — start dose

0.25 mg dose, vial at 1 mg/mL → 0.25 ÷ 1 = 0.25 mL → ×100 = 25 units.

Example 2 — second step

0.5 mg dose, vial at 1 mg/mL → 0.5 ÷ 1 = 0.5 mL → ×100 = 50 units.

Example 3 — same dose, stronger vial

0.5 mg dose, vial at 2 mg/mL → 0.5 ÷ 2 = 0.25 mL → ×100 = 25 units. Same dose as Example 2, half the units.

Example 4 — 1 mg dose

1 mg dose, vial at 2 mg/mL → 1 ÷ 2 = 0.5 mL → ×100 = 50 units.

Example 5 — maintenance dose

2.4 mg dose, vial at 4 mg/mL → 2.4 ÷ 4 = 0.6 mL → ×100 = 60 units.

Example 6 — mg to mcg check

0.25 mg = 250 mcg (×1,000). Useful when a label prints mcg: confirm both numbers describe the same dose before drawing.

Example 7 — why copying units fails

A friend draws "25 units" from a 2 mg/mL vial = 0.5 mg. You draw 25 units from a 1 mg/mL vial = 0.25 mg — half their dose. The mark matched; the dose did not.

Common beginner mix-ups

The number-one mistake is copying someone else's unit count without matching their vial strength, exactly as Example 7 shows. The second is swapping mg and mcg without converting — 1 mg is 1,000 mcg, so a misplaced decimal changes the dose tenfold. The third is treating the 0.25 mg start as a maintenance dose and worrying it "isn't working"; it is simply the bottom of the staircase. When unsure, read the prescription, read the vial, and let the prescriber set the schedule — InjectBuddy only converts the numbers you already have.

So, what is semaglutide?

Semaglutide is a once-weekly injectable GLP-1 medicine that mimics a natural gut hormone, lowering appetite and steadying blood sugar by telling the brain you are full and slowing how fast your stomach empties. Prescribers start the dose low and raise it gradually to limit side effects. It is a prescription medicine, so the right dose is your prescriber’s call; for the units-and-volume side, use the semaglutide units calculator.

Frequently asked questions

What is semaglutide in simple terms?

It is a once-weekly injectable that copies the gut hormone GLP-1. It tells the brain you are full and slows the stomach, which lowers appetite and steadies blood sugar.

Why does semaglutide dosing start so low?

The 0.25 mg start lets the gut adjust and keeps side effects like nausea mild. It is a tolerance step, so the dose steps up about every four weeks toward a maintenance amount.

Is this guide telling me what dose to take?

No. The dose, schedule and whether semaglutide suits you come from a prescriber. This guide explains the concept and the unit maths so the prescribed numbers make sense.

How do milligrams become syringe units?

Divide the dose (mg) by the vial concentration (mg/mL) to get mL, then multiply mL by 100 for a U-100 syringe. So 0.25 mg from a 1 mg/mL vial is 0.25 mL, which is 25 units.

Is semaglutide the same as Ozempic and Wegovy?

Yes — semaglutide is the active ingredient in branded products such as Ozempic and Wegovy, as well as in compounded vials. The molecule is the same; the brand, strength and injection device differ.

Sources

Wilding JPH, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1). N Engl J Med. 2021. PubMed PMID: 33567185.
Kommu S, Whitfield P. Semaglutide. StatPearls. 2024 (mechanism, GLP-1 receptor agonist, ~1-week half-life). NCBI Bookshelf NBK603723.
FDA / DailyMed. WEGOVY (semaglutide) prescribing information — titration from 0.25 mg to 2.4 mg. DailyMed label.
FDA / DailyMed. OZEMPIC (semaglutide) prescribing information — multi-dose pen strengths. DailyMed label.
Jastreboff AM, et al. Tirzepatide Once Weekly for the Treatment of Obesity (comparison context). N Engl J Med. 2022. PubMed PMID: 35658024.

This guide is for general educational purposes only and does not constitute medical advice. Semaglutide is a prescription medicine — always follow your prescriber's specific dose, schedule and instructions.