Published August 1, 2026 · Updated September 4, 2026 · Educational, not medical advice
Most GLP-1 medications are given as a small subcutaneous (just under the skin) injection you can do at home. This page covers both routes in plain English — a pre-filled pen, where the dose is dialled for you, and a vial and syringe, where you draw the amount yourself. The second is where nearly all the serious mistakes happen, so it gets the most space here. It is educational, not medical advice — the product’s own instructions for use and your prescriber’s guidance always come first, and hands-on training from a pharmacist or clinician is worth more than any web page.
The mistake that causes the most harm is a decimal-place mistake, not a technique mistake. The FDA has reported cases of people receiving five to twenty times the intended amount of compounded injectable semaglutide. The most common pattern: someone is told to draw “5 units” from a vial using a U-100 (1 mL) insulin syringe, and draws 50 instead. Reported effects included severe nausea and vomiting, abdominal pain, fainting, dehydration, gallstones and acute pancreatitis. If you are using a vial rather than a pen, read the vial section below carefully and have your pharmacist watch you do it once.
Before you start
Wash your hands and gather everything first: the pen or vial, an alcohol swab, a sharps container, and — for a vial — a clean syringe of the correct type.
Check what you are holding. Confirm the name and the strength printed on the label match what you were prescribed, and check the expiry or beyond-use date. Compounded vials carry a beyond-use date that is usually much shorter than a manufactured product’s expiry.
Look at the liquid. Most GLP-1 solutions should be clear and colourless. Cloudiness, particles, floating specks or a colour change are reasons to stop and call the pharmacy rather than inject.
Temperature. Injecting straight from the fridge stings more for many people. If your product’s instructions allow it to sit out for a short time before use, that can make it more comfortable — but follow the instructions for your specific product rather than a general rule, and never warm it deliberately.
Confirm the amount against your prescription before you touch the pen or the syringe, not while the needle is in your hand.
Where to inject
The three sites commonly used for subcutaneous injection are the abdomen (avoiding roughly a two-inch circle around the navel), the front of the thigh, and the back of the upper arm. Product instructions generally list which sites are approved for that product, and they do not all list the same ones.
Areas to avoid: skin that is bruised, scarred, tender, hardened, broken, tattooed over recently, or lumpy from previous injections. Do not inject through clothing.
Why rotation matters more than people expect. Injecting repeatedly into the same square inch can cause lipohypertrophy — firm fatty lumps under the skin. They are usually painless, which is why they go unnoticed, and the problem is not cosmetic: absorption from a lump is slower and less predictable, so the same amount of medication can behave differently from week to week. Moving to a fresh spot each time avoids this. See injection site rotation for a simple pattern, and tell your prescriber if you can feel any lumps.
Injecting with a pen
A pen holds the medication and lets you dial an amount, so the arithmetic is done for you. The steps below are general; your pen’s own instructions for use are the authority and they differ between products.
Attach a new needle if your pen takes them. A fresh needle every single time — reusing one blunts the tip, which hurts more and damages tissue.
Prime it (sometimes called an air shot or flow check) if the instructions call for it. This clears air from the needle and confirms the pen is working. Skipping it on a pen that requires it can mean part of the dose never leaves the pen.
Dial the amount and check the number in the window before you go near your skin.
Wipe the site with an alcohol swab and let it dry — injecting through wet alcohol stings.
Insert at the angle your pen specifies, pinching a fold of skin if directed, press the button, and hold for the full count the instructions give before withdrawing. This is the step people most often cut short. The medication is delivered over those seconds, and pulling out early leaves some of it on your skin instead of under it.
Withdraw straight out, and put the needle into a sharps container immediately — never recap it.
Injecting from a vial with a syringe
With a vial nothing is pre-measured. You are converting a prescribed amount in milligrams into a number of units on a syringe barrel, and that conversion depends entirely on the concentration of your particular vial. Two vials with the same drug in them can require completely different numbers.
Check the syringe type. A U-100 syringe holds 100 units per mL; a U-40 syringe holds 40 units per mL. The same “10 units” marking means 0.1 mL on one and 0.25 mL on the other — a 2.5× difference. See insulin syringe sizes.
Wipe the rubber stopper with alcohol and let it dry.
Draw the amount, keeping the needle tip in the liquid rather than the air above it. Some people draw a little extra and push back to the exact line.
Clear the large bubbles. Tap the barrel so they rise to the needle end and press them out, then re-check the number against what you wrote down.
Read the barrel at eye level. Looking at a syringe from an angle makes the plunger appear to sit at a different line than it does.
Wipe the site, let it dry, insert at a shallow subcutaneous angle, pinching a fold of skin if you have been shown to, press the plunger steadily to the end, pause a moment, then withdraw and dispose of the whole syringe in a sharps container.
Three things worth checking out loud before every vial injection. They take five seconds and they are the three that produce the FDA-reported overdoses. 1. What is the concentration of this vial? 2. Is this a U-100 syringe or a U-40? 3. Does the number at the plunger match the number I wrote down — and is it a plausible number, or is it ten times bigger than last week’s?
What about air bubbles?
This is the most common worry among people injecting for the first time, and it is largely misplaced. The danger people have in mind — an air embolism — is a concern for injections into a vein. A small bubble in a subcutaneous injection is not considered a safety hazard in the same way.
The real reason to clear bubbles is arithmetic: air in the barrel takes up space that should hold medication, so a large bubble means you receive less than intended. Clear the big ones, do not chase every microscopic one, and check the number after you have cleared them rather than before.
After the injection
A small spot of blood is common. Press gently with clean gauze or a cotton ball. Do not rub the site — rubbing can affect how the medication is absorbed.
A wet patch on the skin suggests some of the dose leaked back out. Do not inject again to make up for it — you cannot know how much was lost, and repeating risks a genuine double dose. Note it and ask your prescriber what they want you to do.
Bruising happens, particularly on the abdomen, and is usually nothing. Frequent bruising is worth mentioning to your clinician, especially if you take a blood thinner.
A missed dose is a question for your prescriber or the product’s instructions, not for a web page — the correct answer depends on the product and on how long it has been.
Log it. Recording what you injected, how much, where and when is the only reliable way to rotate sites properly and to give your clinician an accurate picture at your next appointment.
Common mistakes
Misreading the syringe. Drawing 50 units when 5 were intended is the specific error the FDA has documented, and it is a ten-fold overdose.
Mixing up syringe types. U-40 and U-100 barrels look similar and are not interchangeable.
Confusing milligrams with units. They are different kinds of measurement; the relationship between them is set by your vial’s concentration. See mg, mcg and units.
Skipping the prime on a pen that requires it.
Letting go of the pen button too early, so part of the dose ends up on your skin.
Injecting into the same spot repeatedly, causing lumps that then change how the medication is absorbed.
Reusing or recapping needles.
Injecting cold, wet-with-alcohol skin — a minor comfort issue, but an easy one to avoid.
Disposing of needles and syringes
Used needles are a hazard to household members and to sanitation workers, and loose needles in the bin are illegal in many places. The FDA’s guidance for home users is straightforward:
Put every used needle or syringe straight into an FDA-cleared sharps disposal container — rigid plastic, puncture-resistant, leak-resistant, with a tight-fitting lid. Pharmacies, medical supply companies and online retailers sell them.
Never put loose sharps in household or public rubbish or recycling, and never flush them.
Replace the container when it is about three-quarters full — not when it is packed to the lid.
Disposal rules are set locally, not nationally. Depending on where you live, full containers may be collected from your home for a fee, or dropped off at pharmacies, doctors’ offices, hospitals, health departments, medical waste facilities, or police and fire stations.
The FDA points home users to Safe Needle Disposal on 1-800-643-1643 for the rules in a specific state.
Get hands-on training from your pharmacist or clinician before your first at-home injection, and ask them to watch you do it once even if you have read everything. The product’s instructions for use are the authority. This page is general education, it does not recommend any medication, dose or amount, and it is not medical advice.
Track it in the free app
Log doses, symptoms, weight and reminders on your device — no account, nothing uploaded.
The sites commonly used are the abdomen (avoiding about two inches around the navel), the front of the thigh, and the back of the upper arm. Which of these is approved varies by product, so check your own instructions for use. Avoid skin that is bruised, scarred, tender, broken or lumpy, and move to a different spot each time.
Do I pinch the skin, and at what angle?
Many people pinch a fold of skin and inject at the angle their product specifies, which for subcutaneous injection is usually shallow. Pens and needle lengths differ, so this is one to take from your product’s instructions and from whoever trained you rather than from a general guide.
How do I dispose of needles safely?
Straight into an FDA-cleared sharps container — rigid, puncture-resistant, with a tight lid. Never recap or reuse a needle, never put loose sharps in the rubbish or recycling, and never flush them. Replace the container when it is about three-quarters full and follow your local rules for handing it in. The FDA points home users to Safe Needle Disposal on 1-800-643-1643 for state-specific guidance.
Does site rotation matter?
Yes, and for a practical reason rather than a cosmetic one. Injecting repeatedly into the same spot can create firm fatty lumps called lipohypertrophy, and medication injected into a lump is absorbed more slowly and less predictably. Rotating avoids this. Tell your clinician if you can feel any lumps.
Pen vs syringe — what is different?
A pen is pre-filled and you dial the amount, so no conversion is needed. With a vial you convert a prescribed amount into units on a syringe yourself, and that conversion depends on your vial’s concentration. The injection technique is much the same; the risk profile is not, because the vial route is where dosing errors occur.
Are air bubbles in the syringe dangerous?
The air-embolism risk people are thinking of applies to injections into a vein, not to subcutaneous injections, and a small bubble under the skin is not regarded as a hazard in the same way. The reason to clear the large bubbles is that air occupies space that should hold medication, so a big bubble means you get less than intended. Clear the big ones and re-check the number afterwards.
What if I see blood or the liquid leaks back out?
A small spot of blood is common — press gently with gauze and do not rub. A wet patch suggests some of the dose leaked out; do not re-inject to compensate, because you cannot know how much was lost and repeating risks a real double dose. Note what happened and ask your prescriber.
Should I let the medication warm up before injecting?
Cold liquid stings more for many people. If your product’s instructions permit it to stand at room temperature for a period before use, that can help. Follow the instructions for your specific product, and never warm it deliberately in water, a microwave or on a radiator.
What is the most common serious mistake?
Misreading a syringe. The FDA has reported cases of people receiving five to twenty times the intended amount of compounded injectable semaglutide, most often by drawing 50 units on a U-100 syringe when 5 units were intended. Checking the concentration, the syringe type and the number itself before every vial injection is the habit that prevents it.
Can I inject through my clothes?
No. The site should be clean, visible and wiped with alcohol that has been allowed to dry, so that you can see where the needle is going and check the site afterwards.
U.S. Food and Drug Administration — FDA alerts health care providers, compounders and patients of dosing errors associated with compounded injectable semaglutide products. Source of the five-to-twenty-fold overdose figure, the 5-units-drawn-as-50 pattern, and the reported adverse effects.
U.S. Food and Drug Administration — Safely Using Sharps (Needles and Syringes) at Home, at Work and on Travel, including the sharps-container specification, the three-quarters-full rule, the drop-off site types and the Safe Needle Disposal line.
Manufacturer instructions for use for semaglutide and tirzepatide products (Ozempic, Wegovy, Mounjaro, Zepbound) via DailyMed — approved injection sites, priming and hold-time steps, which differ by product and are the authority for your own.
Standard diabetes injection-technique guidance on site rotation and lipohypertrophy, and on the distinction between air in a subcutaneous injection and air in an intravenous one.
Do this math automatically. Pepathary works out the units to draw, then logs the dose and tracks the vial — offline, on your phone. Get the app.
Pepathary is a personal organizational and educational tool. It is not a medical device, does not diagnose, treat, cure, or prevent any condition, and does not recommend medications or doses. Figures here summarize widely-reported information and general guidance; always confirm what is right for you with your physician or pharmacist.