How to inject peptides safely

How do you give an injection under the skin, step by step?

Wash your hands, wipe the vial’s stopper, draw your dose with a new syringe, inject into a clean spot you didn’t use last time, hold the needle in for 5 seconds, and drop the syringe straight into a sharps container.

1. Wash your handsUse soap and water before you touch the vial or the syringe.^11,12
2. Wipe the vial’s rubber stopperUse an alcohol swab, and let the alcohol dry before the needle goes in.^11,13 Keeping a vial clean
3. Draw a little past your doseWith a new syringe, pull the plunger until its tip passes your dose line. Tap any air bubbles to the top, then push the plunger up until its tip sits on the line.^11,14 Reading a syringe
4. Pick and clean a spotChoose a spot on your belly, thigh or the back of your arm, at least 1 cm from your last shot. Clean it the way your product’s instructions say; if you use alcohol, let it dry first.^3,4,13 Where to inject
5. Put the needle inFollow your prescriber or your product’s instructions on whether to pinch up the skin and at what angle, because advice for 6 mm syringe needles differs.^4,11,15 Needle size and angle
6. Push, then count to 5Push the plunger all the way in, and keep the needle in your skin for at least 5 seconds so the whole dose goes in. If the spot bleeds, press it with gauze; don’t rub.¹¹
7. Straight into a sharps containerDon’t put the cap back on. Drop the whole syringe into a sharps container right away.^7,11 Getting rid of needles

This guide is for shots under the skin with an insulin syringe. It can’t replace being shown in person: FDA encourages asking your prescriber, or the pharmacy that made your drug, how to measure and give your dose.² Technique also can’t fix what’s in the vial. What each kind of vial guarantees

How do you read an insulin syringe?

The lines count units, and on a U-100 syringe 100 units fill 1 mL, so each unit is 0.01 mL. Units measure liquid, not drug: how much peptide is in 10 units depends on how the vial was mixed.

• U-100 means 100 units per mL. It’s the scale on standard insulin syringes: a 1 mL syringe holds 100 units, so each unit is 0.01 mL.^1,14
• Three common sizes. 0.3 mL (30 units), 0.5 mL (50 units) and 1 mL (100 units). The two smaller sizes are usually marked every unit and the 1 mL size every 2 units.^16,17

• Use the smallest size that holds your dose. That’s a syringe maker’s advice, because smaller syringes have finer markings. The dose calculator shows which size fits your draw.¹⁷
• Read at the plunger’s tip. Line up the tip of the plunger, the edge nearest the needle, with your dose line.^11,14
• Units aren’t milligrams. Instructions for compounded drugs often give doses in units, and how much drug is in a unit depends on the vial’s strength. FDA says confusion between mL, mg and units likely contributed to overdoses.² The calculator turns a dose in mg into units for your vial.
• Never swap in a syringe marked in mL. They look alike. ISMP Canada, a medication-safety group, describes a nurse who drew 0.68 mL for a 6.8-unit order, ten times the dose.¹⁹

What dosing mistakes does the FDA see?

Misreading the syringe or miscalculating the dose. In reports behind a 2024 FDA alert, people meant to draw 5 units and drew 50, and overdoses ran from 5 to 20 times the intended dose.

• Prescribers got it wrong too. Converting milligrams to units, some wrote doses 5 to 10 times too high: 0.25 mg, which was 5 units, prescribed as 25 units, and 20 units instead of 2 for three patients.²
• What the overdoses caused. Nausea, vomiting, abdominal pain, fainting, headache, migraine, dehydration, acute pancreatitis and gallstones. Some people needed hospital care. Semaglutide’s half-life is about a week, so an overdose may need watching for longer.²
• “Per mL” isn’t “per vial”. FDA has also documented overdoses when a label’s strength per mL was read as the total in the vial, including 50 times a prescribed dose of fentanyl.²⁰
• Small doses are the hardest to get right. Nurses drawing 2- to 10-unit doses from vials were off by a median of 0.6 to 0.7 units, and by more when they didn’t check for bubbles. On a 2-unit dose, 0.6 units is 30% too much or too little.²¹
• Check before your first dose. FDA encourages asking your prescriber or compounding pharmacy how to measure and give your dose. Bring your dose in mg and the units you plan to draw, so both can be checked.²

What size needle should you use?

The shortest you can get, which on an insulin syringe is 6 mm. Skin is only about 2 mm thick, and needles of 8 mm or longer often reach muscle in the arms and legs of men and slimmer people.

• Skin is thin everywhere. In 388 adults, skin averaged 1.9 to 2.4 mm thick at the arm, thigh, belly and buttocks. The fat under it averaged about 10 to 15 mm and varied far more.²²
• Short needles reach the fat. Going straight in, 4 to 5 mm needles had little risk of reaching muscle in almost all adults. Needles of 8 mm or more may often reach it in the limbs of men and of people with a BMI under 25.²²
• Why muscle matters. A shot into muscle is absorbed faster and less predictably, and can hurt more.^4,16
• What you can buy. Insulin syringes come with 6, 8 or 12.7 mm (½ inch) needles; pen needles go down to 4 mm.^4,16 Injection guidelines make 6 mm the first choice on a syringe, and no longer recommend 12.7 mm for anyone.^4,15,23
• Angle and pinching vary. Advice for 6 mm needles differs: pinch up the skin and go in at 45°, pinch and go straight in at 90°, or no pinch for most people. Follow your prescriber or your product’s instructions.^4,11,15,24

Where should you inject?

Into the fat under the skin of your belly, the front of your thighs or the back of your upper arms, at least 2 inches (5 cm) from your navel. Skip skin that’s bruised, scarred, tender or lumpy.

• Skip damaged skin. Don’t inject where skin is tender, bruised, red, scaly, hard, scarred or damaged, or where it has stretch marks, pits, thickening or lumps.^3,11
• One medicine per syringe and per spot. Zepbound’s instructions say not to mix it with any other medicine, or to inject it in a spot used for other medicines.¹¹

Why rotate injection sites?

Shots in the same spot can leave rubbery lumps of fat under the skin, and medicine injected into them is absorbed less, and less predictably. In insulin users, poor rotation was the biggest risk factor people could change.

• What the lumps are. The medical name is lipohypertrophy: a rubbery thickening of the fat under a much-used spot. It’s often easier to feel than to see.^4,26

• Rotation matters most. Across 51 studies, people who didn’t rotate correctly had about 9 times the odds of lumps, and people who reused needles about 3 times. In one study of 430 people, only 5% of those who rotated correctly had them.^30,31
• Lumps change the dose you get. When 13 people injected insulin into lumps, they absorbed about a fifth less of it, and the amount swung far more from shot to shot.³²
• How to rotate. Put each shot at least 1 cm, about a finger’s width, from the last. One common plan splits each area into quarters and uses one quarter a week.^4,24 GLP-1 drug labels say to rotate within the area you choose, every dose.^3,11
• Never inject into a lump. Injection guidelines are clear on this.^4,23

Are air bubbles in the syringe dangerous?

Under the skin, a small bubble isn’t the danger: a syringe maker says bubbles aren’t dangerous, and some FDA-approved prefilled syringes are injected with theirs. The problem is the dose, because air takes up room meant for medicine.

• What the syringe maker says. embecta, which makes BD insulin syringes, says bubbles aren’t dangerous but can make the dose less accurate.²⁴
• Some labels say to leave them. The instructions for Lovenox’s prefilled syringe and Wegovy’s say not to remove the air bubbles; Wegovy’s add that they don’t affect the dose in that syringe.^3,33
• From a vial, get them out. Tap the syringe so bubbles rise to the top, then push the plunger up to your dose line.^11,15
• Unchecked bubbles mean dosing errors. In the study of nurses drawing small doses, errors were larger when bubbles weren’t checked.²¹

How do you keep a vial clean, and how long does it last once opened?

Wipe the stopper with alcohol and use a new needle and syringe for every draw. Throw an opened vial away after 28 days unless its maker gives another date.

• A new needle for every draw. FDA says reusing a needle or syringe can contaminate the medicine left in the vial.^5,34
• Never leave a needle in the stopper. Leaving one in for repeat draws gives germs a direct route into the vial.⁶
• Preservatives have limits. The preservative in a multi-dose vial doesn’t stop viruses and doesn’t fully protect against bacteria.⁶
• The 28-day rule. CDC says to date an opened multi-dose vial and throw it away within 28 days, unless its maker gives another date. FDA says the same for compounded vials, even if medicine is left.^5,6 Approved drugs set their own limits: an opened Zepbound multi-dose vial goes after 30 days or 4 weekly doses, even with medicine left, and an opened vial of Humulin R after 31 days.^11,14
• Mixed research peptides have no official limit. Bacteriostatic water’s label says not to store a drug after mixing unless that drug’s maker says how. Peptides sold for research come with no such instructions.³⁵
• Bacteriostatic water itself. It’s sterile water with 0.9% or 1.1% benzyl alcohol as a preservative, for mixing other drugs only. Keep it at room temperature, 20–25 °C (68–77 °F), use it only if it’s clear and the seal is intact, and never give it to a newborn.³⁵
• When in doubt, throw it out. CDC says to discard any vial whose sterility is in doubt, and FDA advises not using an injectable GLP-1 that arrives warm.^5,6
• Never share. Syringes, pens and cartridges are for one person only, even with a new needle.³⁶

Can you reuse a needle or syringe?

No. Drug instructions and the CDC say to use a new one for every shot. A used needle isn’t sterile, can contaminate your vial, and is linked to lumps under the skin and more painful shots.

• Labels say use it once. A used needle is no longer sterile and can clog.^11,16
• More lumps. In 430 insulin users, the risk of lumps rose when a needle was used more than 5 times. In a 2024 trial of 71 people, reusing syringes 5 times modestly increased lumps within 12 weeks.^31,37
• More pain. Wiping a needle with alcohol to reuse it strips the coating that helps it glide, so shots hurt more.²⁶
• Weak evidence isn’t reassurance. Studies of insulin users haven’t shown more skin infections from reuse, but a review found all 25 studies at high risk of bias. And a reused needle that goes back into a vial can contaminate what’s left.^5,38

How do you get rid of used needles?

Put each one straight into a sharps container, and when it’s about three-quarters full, get rid of it the way your area’s rules say. Never put loose needles in the trash, the recycling or the toilet.

• Use an FDA-cleared sharps container. Pharmacies, medical supply companies, health care providers and online stores sell them. Ask your insurer whether it covers one, or the drug’s maker whether it gives them away.^7,39 Our supplies list includes one.
• No container? A heavy plastic bottle can do. Something like a laundry detergent bottle works if it’s heavy-duty plastic with a tight, puncture-resistant lid, stays upright, doesn’t leak, and is labeled as hazardous waste.⁸
• Stop at three-quarters full. Overfilling raises the risk of a needlestick. Don’t reuse the container, and keep it away from children and pets.^8,40
• Where it goes depends on where you live. Options include drop boxes or supervised collection sites, household hazardous waste sites, mail-back programs and special-waste pickup. Safe Needle Disposal, at 1-800-643-1643, has each state’s rules.⁴⁰
• Never loose in the trash, recycling or toilet. Loose needles can injure people and pets and spread hepatitis B, hepatitis C and HIV.⁷
• Don’t recap. Drug instructions say not to put the cap back on, because that’s how needlesticks happen. If you must, FDA describes a one-handed method.^11,41
• If someone else’s used needle sticks you. Wash the spot right away with soap and water, or an antiseptic such as rubbing alcohol or hand sanitizer, and get medical care right away.⁷
• Flying with needles. TSA allows unused syringes with injectable medicine and used ones in a sharps or other hard-sided container; declare unused syringes and liquid medicine at the checkpoint. FDA suggests carrying a travel-size sharps container.^40,42,43,44

How can you tell if an injection site is infected?

A spot that’s red, swollen, warm, painful or draining pus, especially with a fever, needs a doctor. Get emergency care for signs of sepsis such as confusion, a racing heart or trouble breathing.

• What CDC says to look for. A bump or area that’s red, swollen, painful, warm to the touch, full of pus or draining, or that comes with a fever. Contact a doctor, especially with a fever or if it hasn’t improved within 48 hours.⁹
• Don’t squeeze it. Don’t pick at or pop the bump. Cover it with clean, dry bandages until you’re seen.⁹
• Two common infections. An abscess is a red, raised, painful pocket of pus, usually drained and treated with antibiotics. Cellulitis brings painful red skin, fever, chills and swollen glands.^45,46
• Sepsis is an emergency. A skin infection can lead to sepsis. Signs include clammy or sweaty skin, confusion, extreme pain, fever or shivering, a fast heart rate or weak pulse, and shortness of breath.¹⁰
• Fake products cause reactions too. FDA describes a report tied to fake “compounded tirzepatide” with redness, swelling, pain and a red lump where it was injected.⁵

Not sure whether a reaction is serious? Our guide to side effects sorts symptoms into call 911, get care today, and tell your doctor.

Sources

Steps come from FDA-approved drug instructions, the CDC, the FDA and injection-technique guidelines; numbers come from the studies listed. Tap a number anywhere on this page to see its source.