A Mounjaro injection goes under the skin, once a week, on the same day each week, into the abdomen, the front of the thigh, or the back of the upper arm if someone else is doing it. The Mounjaro injection itself is straightforward, and most of what goes wrong involves storage, timing or lifting the pen too early rather than anything about the needle.
Always follow the Instructions for Use that came with your prescription and whatever your clinician or pharmacist showed you. This page is background, not a substitute for either.
Mounjaro injection sites: where to inject
Three sites are approved, all of them places with a reliable layer of fat under the skin for the medicine to absorb from.
| Site | Best for | How to use it | What to avoid |
|---|---|---|---|
| Abdomen | Most self-injections | Stay at least two inches away from the navel | Scars, bruises, hard or lumpy areas, the navel itself |
| Front of thigh | People who want to see the site clearly | Use the middle of the front of the thigh | The inner thigh, anywhere near the knee, irritated skin |
| Back of upper arm | When another person is injecting | Someone else injects into the fatty area at the back | Attempting to twist round and do it yourself |
Rotation does not need a chart. Abdomen left, abdomen right, thigh left, thigh right, then back to the start is enough. The only thing that really matters is not using the same spot two weeks running, which is what leads to lumps and patchy absorption over time.
The general principle applies to any weekly subcutaneous injection: spread the load across the available skin.
Using the pen
The Mounjaro pen hides the needle, which removes the part most people dread. The sequence is short.
- Check it. Right medicine, right dose, in date, and the liquid clear and colourless to slightly yellow.
- Let it warm. Taking it out of the fridge a few minutes early makes the injection noticeably more comfortable. A cold pen stings.
- Wash your hands, clean the site, and let the alcohol dry completely. Wet alcohol stings.
- Pull off the base cap. Do not unlock the pen before it is against your skin.
- Press the base flat against the site and keep firm pressure.
- Unlock, press and hold. The first click means the injection has started. The second means it has finished.
- Keep holding for up to ten seconds after the second click. This is the step people skip, and lifting early means part of the dose ends up on your skin.
- Dispose of the pen in a sharps bin, not in household waste.
You can inject at any time of day, with or without food. Consistency of day matters more than time.
Missed doses
If you realise within four days of the scheduled day, take the dose and carry on with your normal schedule.
If more than four days have passed, skip it and take the next one on your usual day. Do not double up to compensate.
If you need to change which day of the week you inject, at least three days must pass between doses. See our missed tirzepatide dose page for the detail.
Storage
In the fridge, at 2 to 8 degrees C, until the expiry date on the pen. Do not put it in the freezer compartment or against the back wall where things freeze.
At room temperature, up to 30 degrees C, for a maximum of 21 days. That window is cumulative, not a fresh 21 days each time it comes out of the fridge, and once a pen has been out it should not go back in for long-term storage.
Never use a pen that has been frozen, even if it thawed and looks normal. Freezing damages the protein and there is no way to see it.
Also discard a pen that is cloudy, discoloured, has visible particles, or was dropped onto a hard surface, since the internal mechanism can be damaged without any visible sign.
What to expect afterwards
A small amount of redness, itching or a light bruise at the site is common and settles within a day or two. Rotating sites reduces it.
What is not routine: a reaction that spreads and worsens over 24 to 48 hours, warmth with fever, or any hives, facial swelling or difficulty breathing, which needs emergency care.
The gastrointestinal effects, nausea in particular, are about the drug rather than the injection and tend to be worst in the days after a dose increase. Our Mounjaro side effects page covers those.
Common mistakes
Lifting the pen after the first click. The first click is the start, not the finish. Waiting for the second and then holding is the single most common error.
Injecting into muscle. Mounjaro is subcutaneous. Very lean people injecting into the thigh at a poor angle can reach muscle, which stings more and changes absorption. Pinching a fold of skin helps.
Reusing the same spot. Repeated injection in one area causes lumpy fatty tissue that absorbs unpredictably.
Leaving the pen out too long. The 21 day room temperature allowance is a total, and people who take a pen travelling and bring it home often lose track.
Injecting a cold pen straight from the fridge. It works, it just hurts more than it needs to.
If you are new to it
The first injection is the one people worry about, and it is worth having a pharmacist or nurse watch you do it once. Most find the pen easier than expected, since there is nothing to see and nothing to aim.
For dosing schedules and titration, see our tirzepatide dosage chart, and for the drug generally the Mounjaro guide.
FAQ
Where is the best place to inject Mounjaro?
The abdomen is the most practical for self-injection, staying at least two inches from the navel. The front of the thigh works if you prefer to see the site. The back of the upper arm is approved but needs another person to do it.
What happens if I miss a Mounjaro dose?
Take it if you realise within four days of your usual day, then continue as normal. If more than four days have passed, skip that dose and take the next one on schedule. Never double up.
How long can Mounjaro stay out of the fridge?
Up to 21 days at room temperature, below 30 degrees C, and that is a cumulative total rather than a fresh window each time. A pen that has been out should not be relied on for long-term fridge storage afterwards.
Does the injection hurt?
Usually very little. The needle is fine and hidden, and most discomfort comes from injecting a cold pen or from alcohol that has not dried. Taking the pen out a few minutes early helps more than anything else.
Can Mounjaro be injected into muscle by mistake?
It can, particularly in the thigh in someone with little subcutaneous fat. It stings more and may alter absorption. Pinching a fold of skin and keeping the pen flat against the site prevents it.
Does it matter what time of day I inject?
No. Any time works, with or without food. What matters is keeping to the same day each week, since the drug is dosed weekly and a consistent interval keeps levels stable.
