Dosing & Usage

How to Inject Weight Loss Injections

Learn how to inject Mounjaro or Wegovy step-by-step, including preparation, technique, and tips to make your first injection easier.

Whether you are about to give yourself your first weight-loss injection or want to refresh your technique a few weeks in, this guide covers every step of administering Mounjaro or Wegovy at home — preparing the pen, choosing and rotating injection sites, injecting safely, and disposing of used pens. The instructions are based on the manufacturer guidance in the patient information leaflets and the SmPCs. LetsLoseWeight is an independent comparison site; we do not prescribe medication, and the instructions inside your specific pen pack take precedence over anything written here.

Before you start: what you'll need

For each injection you should have:

  • The pre-filled pen — at the dose your prescriber has specified
  • An alcohol swab to clean the injection site (most pharmacies include these)
  • A sharps bin for disposal of the used pen — most regulated UK pharmacies will provide one
  • Clean hands — wash with soap and water before starting
  • A clean, well-lit space to inject in

Preparing the pen

Take it out of the fridge

Pens should be stored in the fridge at 2–8°C. Take the pen out of the fridge 15–30 minutes before injecting to let it warm to room temperature — a cold injection is more uncomfortable for many people. Do not warm it artificially (e.g. with hot water or a microwave).

Inspect the pen

Before each injection:

  • Check the expiry date on the pen label
  • Check the medicine through the inspection window — it should be clear and colourless
  • Do not use if the medicine is cloudy, discoloured, or contains particles
  • Do not use if the pen is damaged, frozen, or has been exposed to high temperatures

Confirm the dose

Check the dose printed on the pen matches what your prescriber has prescribed. Mounjaro and Wegovy have multiple dose strengths — using the wrong one can mean either a much higher dose than intended or no effective dose at all.

Choosing where to inject

Both Mounjaro and Wegovy are given as subcutaneous injections — into the layer of fat just under the skin, not into a muscle or vein. The licensed injection sites are:

  • The abdomen — at least 5 cm (about 2 inches) away from the belly button
  • The front of the thigh — the central area, away from the inner and outer edges
  • The back of the upper arm — the fatty area, often easiest to reach with help

Rotate sites

Using the same exact spot week after week causes:

  • Lumps under the skin (lipohypertrophy)
  • Reduced absorption of the medicine
  • Bruising and skin irritation

Rotate between the three areas (abdomen one week, thigh the next, etc.) and within each area (different spots within the abdomen). A simple system: imagine the abdomen as a clock face and inject at a different hour each week.

For more on injection sites and rotation, see our where to inject guide.

How to inject — step by step

The exact steps depend on the specific pen design. Mounjaro and Wegovy use slightly different pen mechanisms; both come with detailed pictorial instructions inside the pen pack. The following is a general overview — always follow the instructions inside your specific pen pack.

1. Wash and prepare

Wash your hands with soap and water. Choose your injection site and clean it with an alcohol swab. Let it air-dry for a few seconds — injecting through a wet swab stings more.

2. Remove the cap

Pull the cap straight off the pen. Do not twist it. Do not touch the needle.

3. Place the pen against the skin

Hold the pen perpendicular to the skin (90° angle, straight in). The pen should be flat against the skin with the needle covered by the safety guard.

4. Press and hold

Press the pen firmly against the skin. The injection mechanism activates when the pen is pressed down properly. You should hear a click, and the dose indicator window will start to move.

Hold the pen in place for the time specified in your pen's instructions — usually 5–10 seconds. This ensures the full dose is delivered. Lifting the pen too early can mean an incomplete dose.

5. Listen and watch for confirmation

You should hear a second click when the injection is complete, and the dose indicator should show the dose has been fully delivered. Some pens have a window that turns a different colour when complete.

6. Remove the pen

Lift the pen straight up from the skin. The needle safety guard will lock automatically — you should not be able to push it back down.

7. Dispose of the pen

Place the used pen directly into the sharps bin. Do not recap the pen. Do not put pens in the regular bin.

Common technique mistakes

Things that go wrong and how to avoid them:

  • Injecting too quickly — lift-off before the full dose is delivered. Hold for the full count specified.
  • Injecting at an angle — should be 90° straight in, not slanted.
  • Pinching too hard — gentle skin pinch (or no pinch in many cases — check your pen instructions). Squeezing too hard can push fat away from the needle.
  • Wrong site — avoid scarred areas, moles, broken skin, the belly button area, the inner thighs, joints, and any area with active eczema or psoriasis.
  • Cold pen — let it warm to room temperature before injecting.
  • Reusing pens — Mounjaro and Wegovy pre-filled pens are single-use. Each pen is meant for one injection only.

What to do if injection goes wrong

Bleeding or bruising

A small amount of bleeding or a small bruise is normal and not concerning. Press gently with a clean tissue or cotton wool — do not rub. Mild bruising will fade in a few days.

Pen click without medicine

If you don't see the dose indicator move, or you suspect the medicine has not been delivered, do not redose. Contact the pharmacy that supplied the pen — they may advise based on what happened.

Medicine leaks at the injection site

A small drop is normal and means a small amount of medicine has come back to the surface. Most of the dose has still been delivered. Do not redose.

Severe pain at injection site

A small sting or bruise is normal. Severe, prolonged pain is not. Stop injecting if it occurs and contact your prescriber.

Storage and disposal

Storage

  • Unused pens: in the fridge at 2–8°C
  • In-use pen (Wegovy): up to 28 days at room temperature (below 30°C) once first used
  • Out-of-fridge (Mounjaro): up to 21 days at room temperature (below 30°C)
  • Never freeze — frozen pens must be discarded
  • Protect from light — keep pens in their original packaging until use

Disposal

  • Used pens go into a sharps bin — never the regular bin or recycling
  • A full sharps bin can be returned to most UK community pharmacies for safe disposal
  • Do not recap used pens before disposal — the safety guard locks automatically

Inject on the same day each week

Both Mounjaro and Wegovy are weekly injections. Picking a fixed day:

  • Helps establish a routine and reduces missed doses
  • Keeps the medicine's blood levels more consistent
  • Makes dose-change tracking simpler

The injection can be at any time of day, with or without food. Some people prefer evening (so any nausea is overnight); others prefer morning. Pick what works for you and stick to it.

If you need to change your injection day, the medicines allow some flexibility — see our missed dose guide for the rules.

Special situations

If you are unwell

If you are seriously unwell — particularly with severe vomiting, diarrhoea or signs of dehydration — talk to your prescriber before your next injection. They may advise pausing for a week.

Travel

Mounjaro and Wegovy can be transported. For travel:

  • Short trips (under 21–28 days): room-temperature storage is fine within the out-of-fridge limits
  • Longer trips: consider a small insulated cool bag with a temperature monitor
  • Flying: pens can be carried in hand luggage. Bring a copy of your prescription. Check airline rules; some require advance notice for needles
  • Customs: bring documentation showing the medicine is yours and prescribed

Pregnancy or breastfeeding

Stop the medicine and contact your prescriber. Both Mounjaro and Wegovy are not licensed in pregnancy or breastfeeding.

Frequently asked questions

Does the injection hurt?
Most people find it minimally uncomfortable — a brief pinch or sting. Letting the pen warm to room temperature, choosing a fleshier injection site, and good technique all reduce discomfort.

Can I inject with someone watching/helping?
Yes — many people find help easier, particularly for upper-arm injections that are difficult to do solo. The person helping does not need to be a clinician.

What if I forget to inject on my usual day?
Mounjaro can be taken up to 96 hours late on the same day; Wegovy up to 48 hours late. Beyond those windows, skip the missed dose. See our missed dose guide.

Can I shower or swim after injecting?
Yes — once any small bleeding has stopped (a few minutes), normal activity is fine. There are no restrictions.

My pen looks slightly different from a video I watched online.
Pen designs are updated periodically by manufacturers. Always follow the instructions inside your specific pen pack rather than third-party videos.

Next steps

Sources

This guide is for general information only. The instructions inside your specific pen pack take precedence. If you are uncertain about technique, ask your supplying pharmacy for guidance.

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How to Inject Weight Loss Injections UK: Full Guide | LetsLoseWeight