So you have seen the words “retatrutide 40mg pen” and thought: what is that, why 40mg, how many doses do I get, and is the one I am looking at even real? Good — those are exactly the right questions. This guide answers all of them in plain English: the pen format, how the click system works, dosing and titration context, what a fair UK price looks like, how to read a COA, and what every researcher should check before ordering. If you are starting from the basics, we also have a UK guide on what do peptides do. No jargon soup, no lab-coat lecture.
One note up front: retatrutide is a research peptide, supplied for laboratory research only and not for human use in the UK.
Key takeaways (the 30-second version)
- A retatrutide 40mg pen is a pre-filled pen holding 40 milligrams of retatrutide in total — a triple-receptor research peptide (GLP-1, GIP and glucagon).
- The 40mg is the whole tank, not one dose. How long it lasts depends on the dose size you draw.
- On a Retatrutide 2.0 pen, 3 clicks = 0.5mg and 6 clicks = 1mg — so a 40mg pen has roughly 240 clicks in it.
- A pen means no mixing, no bac water, no syringes — the easy-mode format next to a vial.
- The only thing that proves what is inside is a batch-specific third-party COA (think Janoshik), backed by a hologram and unique batch code you can check the second it arrives.
- UK 40mg pens usually sit around £150–£180. Buy on cost per verified milligram, not the lowest sticker.
Short version: the only thing that separates a real pen from an expensive fake is proof you can check — and that is exactly what MyReta is built to solve.
Table of contents
What is a retatrutide 40mg pen?
Retatrutide is a peptide that pokes three receptors at once: GLP-1, GIP and glucagon. Most weight-research peptides only hit one or two. Retatrutide hits all three, which is why scientists call it a “triple agonist” and why it gets so much attention. A 2023 Phase 2 trial in the New England Journal of Medicine is the study most people are talking about.
A 40mg pen is that peptide, already mixed and sealed inside a pre-filled injector pen, with 40 milligrams of it in total. Think of it like a marker pen with a dial: the ink is already inside, and you set how much comes out. No bottle to open, no powder to mix, no separate syringe of water to add.
Why does 40mg matter? Because it is one of the larger, better-value pen sizes on the UK research market. A bigger tank usually means a lower price per milligram and fewer reorders. It is the sweet spot most researchers land on. For the wider science and the UK legal picture, our main Retatrutide UK page has you covered; this guide stays laser-focused on the pen.
Retatrutide 40mg pen vs vial: what is the difference?
Retatrutide comes two ways: as a pen (pre-filled, ready) or as a vial (a little bottle of dry powder you mix yourself). Both can be excellent. They just ask different things of you.
With a vial you have to reconstitute it — add bacteriostatic water, work out the right amount, swirl gently, and draw each dose with an insulin syringe while doing a bit of mental arithmetic every time. A pen skips all of that: it is pre-mixed, and you dial the dose. Here is the honest side-by-side:
| What matters | Pre-filled pen | Vial + bac water |
|---|
| Format | Ready to use, sealed | Dry powder you mix |
| Ease of use | High — dial and go | Lower — mixing required |
| Measuring / dosing | Click system does the maths | You measure each dose by hand |
| Storage | Fridge, simple | Fridge, plus bac-water handling |
| COA availability | Should come with batch COA | Should come with batch COA |
| Cost per mg | Slightly higher | Often lower |
| Best for | Simplicity, consistency | Flexibility, lowest cost per mg |
New to this? The pen is the gentler start. If you do choose a vial, our how to reconstitute retatrutide guide walks the mixing step-by-step.
Retatrutide dosing: how the 40mg pen is usually understood
This is not a dosing plan for a person — it is how dosing is understood in the research context, so the pen maths makes sense.
Three things decide what a “dose” means on a pen:
- Concentration — how much peptide is packed into the liquid.
- The click system — how many clicks the device gives per milligram (more on this below).
- The intended research protocol — what the study is set up to look at.
In the Phase 2 research, retatrutide was given once weekly and titrated — started low and stepped up slowly so the gut had time to settle. A common pattern: 2mg to begin, then 4mg, then 8mg, up to 12mg in some arms — a research reference, not a recommendation. Always follow the supplier’s device guide and the COA.
How many doses are in a 40mg retatrutide pen?
This is the question almost no one answers properly, so here it is in plain numbers. 40mg is the total in the pen, not one dose. How many doses you get is just simple division:
| If each dose is… | Doses in a 40mg pen | Roughly how many weeks (once weekly) |
|---|
| 0.5 mg | 80 doses | ~80 weeks |
| 1 mg | 40 doses | ~40 weeks |
| 2 mg | 20 doses | ~20 weeks |
| 4 mg | 10 doses | ~10 weeks |
| 8 mg | 5 doses | ~5 weeks |
| 12 mg | about 3 doses | ~3 weeks |
So a 40mg pen is a lot of small doses or a few big ones. Once you picture it as a fuel tank, the number stops being scary.
Retatrutide pen guide: how the click system works
Most research pens use a click dial. You turn the end of the pen and it clicks, one notch at a time. Each click pushes out a tiny, measured amount — so instead of measuring liquid by eye, you just count clicks.
On the Retatrutide 2.0 pen, the click system is straightforward:
- 3 clicks = 0.5 mg
- 6 clicks = 1 mg
From there the maths is easy — every 6 clicks adds another milligram:
| Target dose | Clicks |
|---|
| 0.5 mg | 3 clicks |
| 1 mg | 6 clicks |
| 2 mg | 12 clicks |
| 4 mg | 24 clicks |
| 8 mg | 48 clicks |
| 12 mg | 72 clicks |
Here is a neat way to picture the whole pen: at 6 clicks per milligram, a 40mg pen holds about 240 clicks in total. That is your full tank, counted out in little notches.
Two honest caveats. First, the number of clicks per milligram depends on the specific device and its concentration — the figures above are for the Retatrutide 2.0 pen, so always check the guide that comes with your pen. Second, a pen is built around three simple moves: dial (turn to your dose), prime (push out a tiny test amount first to clear air, like flicking a hose until it runs clean), and dispense (press and hold). The leaflet in the box is the boss — follow it.
How long does one 40mg pen last?
Because research dosing is usually stepped up over time, a single 40mg pen does not last a fixed number of weeks — it depends on the ramp. Here is a worked example using a typical step-up, just so you can see the maths:
| Weeks | Dose each week | Used in this block | Left in the 40mg pen |
|---|
| 1–4 | 2 mg | 8 mg | 32 mg |
| 5–8 | 4 mg | 16 mg | 16 mg |
| 9–10 | 8 mg | 16 mg | 0 mg |
Add it up and one 40mg pen covers roughly the first 10 weeks of a standard step-up. Hold a steady low dose and it stretches much further; jump straight to big doses and it empties faster. Want to plan your own numbers without doing sums on the back of an envelope? That is exactly what a dosing calculator is for — ask us and we will point you to ours.
Retatrutide 40mg pen price UK 2026
Most UK 40mg pens land around £150–£180. To compare sensibly, look at both the price per pen and the price per milligram — bigger pens usually win on the latter, the same way a family-size cereal box does:
| Pen size | Typical UK price | Rough price per mg | Good for |
|---|
| 20 mg | ~£120 | ~£6.00 | Trying it out, short studies |
| 30 mg | ~£149 | ~£4.97 | Middle ground |
| 40 mg | ~£165–£175 | ~£4.13–£4.38 | Best all-round value |
| 50 mg | ~£176–£220 | ~£3.52–£4.40 | Longer research runs |
Why cheap is not always better. A £130 pen with no COA is not cheap — it is a mystery box with a price tag. If it is under-filled, or it is not even retatrutide, your “cost per real milligram” is basically infinite, because you got nothing you can trust. When you weigh up price, weigh up the whole package: COA, purity, third-party testing, storage and cold-chain, customer support, and supplier reputation. A slightly dearer pen that ticks those boxes is the cheaper buy the moment you count what you actually received. We make the full case in our cheapest retatrutide UK breakdown and price guide.
What should a retatrutide COA show?
A Certificate of Analysis (COA) is the lab report that proves what is in your batch. A shiny pen tells you nothing — the label is just a sticker. A COA worth trusting shows all of this:
| On the COA | What you want to see |
|---|
| Compound identity | Confirmed as retatrutide (not a cheaper stand-in) |
| Purity | ≥99% |
| Batch / lot number | Present, and matches your pen |
| Test date | Recent and clearly dated |
| Testing lab | Independent third party (e.g. Janoshik) |
| HPLC / LC-MS | HPLC for purity, mass spec for identity |
| Endotoxin / microbial | Reported where available — a sign of a careful supplier |
For a deeper walkthrough of how to read each number, see how to verify a peptide COA in the UK, plus our own COA page and quality testing page.
How to check if a 40mg pen is legit
You do not need to be an expert to smell a dodgy pen. Run down this red-flag list — if you tick more than one or two, walk away:
- No COA — or only a blurry image with no lab named.
- No batch number, or one that does not match the pen.
- Unrealistic claims (miracle results, “100% guaranteed”).
- No storage guidance — a real supplier tells you to keep it cold.
- No clear concentration or click guide.
- No supplier details — no company, no contact, no returns.
- Suspiciously low pricing — if it is way under the market, ask why.
The best suppliers make this easy: a tamper-evident hologram and a unique batch code on the back of the box that you match to the COA the moment it arrives. If a seller gets twitchy when you ask to verify a batch — that twitchiness is your answer.
Retaklik 40mg vs retatrutide 40mg — what is the difference?
This one trips everyone up, so let us clear it fast. Retatrutide is the compound — the actual peptide. Retaklik is a brand name used for retatrutide products. So a “retaklik 40mg” pen and a “retatrutide 40mg” pen are talking about the same core compound, just under different labels. It is a bit like “hoover” and “vacuum cleaner.” Our Retaklik UK guide tells the full brand story, and the homepage covers Retaklik 40mg and Retatrutide UK in one place.
Storage and handling
Peptides are a bit fussy, but the rules are easy:
- Keep it cold. Fridge temperature, around 2–8°C. Treat it like fresh milk, not tinned beans.
- Never freeze it. Freezing can wreck the peptide — keep it off the ice shelf.
- Do not shake it. A gentle life is a happy peptide.
- Keep it out of bright light, capped, and away from kids and pets.
- Check the expiry and follow the supplier’s cold-chain instructions — do not use a pen that has been dropped or looks damaged.
Side effects and safety: what the research shows
Retatrutide is a research compound, not a treatment. So this is a plain-English round-up of what trials and the wider GLP-1 family have reported — not advice for anyone to dose themselves.
Common, mild effects seen in trials
Most reported effects were tummy-related. They mostly showed up during the step-up weeks and then calmed down as the body got used to it:
- Feeling sick (nausea)
- Being sick (vomiting)
- Diarrhoea or constipation
- Less hunger
- Feeling tired
Where an injection is involved, the spot can react too — a bit of redness, mild swelling, bruising, itching, or a small firm lump if the same spot is used again and again. These are usually minor and fade in a few days. Rotating the spot each time helps.
When to get help fast
Some signs are not “wait and see”. Get medical help quickly for any of these:
- Bad or lasting tummy pain (a possible sign of pancreatitis)
- Signs of an allergy — rash, a swollen face, or trouble breathing
- Pain in the upper-right belly, fever, or yellow skin (a possible gallbladder problem)
- Lots of vomiting or diarrhoea with dizziness or very little wee (dehydration)
- A racing heart, dizziness, or fainting
- New or changing eyesight problems, especially with diabetes
In the UK, call 999 for anything severe or life-threatening, or NHS 111 for urgent advice. A suspected bad reaction to a medicine can be reported through the MHRA Yellow Card scheme.
Extra care points for this family of compounds
Researchers flag a few areas of caution for incretin-class compounds: a past history of pancreatitis or gallbladder trouble, thyroid worries, pregnancy or breastfeeding, and use alongside insulin or similar medicines (which can push blood sugar too low). This class can also slow how fast the stomach empties, which may change how other medicines are taken up. That is background from the literature, not dosing advice.
To be crystal clear: retatrutide is not licensed for human use in the UK. Anyone looking for a real weight or metabolic treatment should talk to a GP or pharmacist about approved options like semaglutide or tirzepatide.
Retatrutide 40mg pen UK: who is it for?
Plainly: this is for research customers, not for medical treatment. Retatrutide pens are bought by people studying the compound — its triple action on the GLP-1, GIP and glucagon receptor pathways, and how those pathways behave in metabolic research. If you are looking for a treatment for a person, this is not that, and a pharmacy or GP is the right door. If you are a researcher who wants a verifiable, pre-filled, easy-to-handle format, a 40mg pen is a sensible pick.
Where to get a lab-tested retatrutide 40mg pen
If you want a 40mg pen you can actually verify, that is the whole reason MyReta exists. Our Retatrutide 2.0 40mg pen ships with a batch-specific third-party COA, plus the tamper-evident hologram and unique batch code on the box so you can check it the second it arrives — and it uses the simple click system above (3 clicks = 0.5mg, 6 clicks = 1mg).
Heads up: the 40mg pen is currently out of stock — we are restocking within about a week. Keep an eye on the product page for the restock, and meanwhile our where to buy retatrutide UK guide covers what to check before you buy from anyone. For the full range and the science, start at the Retatrutide UK homepage.
Frequently asked questions
Is retatrutide available in the UK?
Yes, as a research peptide for laboratory use only. It is not a licensed medicine in the UK and is not approved for human use, so it cannot be prescribed or sold for treatment.
Is a 40mg pen better than a vial?
For ease of use, yes. A pen is pre-mixed, so there is no reconstitution and no measuring maths — you just count clicks. A vial is often cheaper per milligram but you mix and measure it yourself.
How many doses are in a 40mg retatrutide pen?
It depends on dose size: 80 doses at 0.5mg, 40 at 1mg, 20 at 2mg, 10 at 4mg, 5 at 8mg, or about 3 at 12mg. The 40mg is the total in the pen, not one dose.
How long does a 40mg pen last?
On a typical research step-up (2mg, then 4mg, then 8mg) one 40mg pen covers roughly the first 10 weeks. Held at a steady low dose it lasts much longer; used at high doses it empties faster.
How does the click system work on a Retatrutide 2.0 pen?
3 clicks equals 0.5mg and 6 clicks equals 1mg, so every 6 clicks adds another milligram (12 clicks = 2mg, 24 = 4mg, and so on). A 40mg pen holds about 240 clicks. Always check the guide for your specific pen.
What does COA mean?
COA stands for Certificate of Analysis — a lab report confirming a batch’s identity (by mass spec) and purity (by HPLC). It is the only real proof of what is inside your pen.
What purity should I look for?
Aim for ≥99% purity by HPLC on a batch-specific, third-party COA. Lower figures can mean synthesis by-products or off-target material.
Is retatrutide the same as tirzepatide?
No. Tirzepatide is a dual agonist (GLP-1 and GIP). Retatrutide is a triple agonist, adding glucagon as a third target. They are different compounds.
Does retatrutide need to be refrigerated?
Yes. Store it cold, around 2–8°C, out of light, and never freeze or shake it. Follow the supplier’s cold-chain instructions.
Why is retatrutide expensive?
It is a complex triple-agonist peptide, and quality suppliers add third-party testing, cold-chain handling and verification. Cheaper, untested products often cost more in the end because you cannot trust what is inside.
Can you buy retatrutide online in the UK?
You can buy it online as a research peptide for laboratory use only. It is not sold for human use. Always check the COA, batch details and supplier transparency first.
What should I check before buying a 40mg pen?
Check the concentration and click guide, the batch-specific COA (≥99% purity, independent lab), the batch number and hologram, storage instructions, and clear supplier details. If any are missing, treat it as a red flag.
Where is retatrutide injected, and how often?
In the trials it was given once a week as a small jab under the skin (subcutaneous), usually in the belly or the outer thigh. The spot is swapped each time so the same patch of skin does not get sore or lumpy. This is research background, not advice to inject anyone.
Can you join a retatrutide clinical trial in the UK?
You can look for live studies on the NHS “Be Part of Research” site (bepartofresearch.nihr.ac.uk) or on ClinicalTrials.gov. You may need a GP or specialist to refer you and check if you can take part.
Scientific references
- Triple–Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial (NEJM, 2023)
- Retatrutide for people with type 2 diabetes — a Phase 2 trial (The Lancet, 2023)
- LY3437943 (retatrutide), a triple GIP/GLP-1/glucagon agonist — Phase 1b trial
- MHRA: avoid illegal online weight-loss medicines
- MHRA Drug Safety Update: GLP-1 / GIP agonists — pancreatitis warnings
- NICE TA875: Semaglutide for overweight and obesity
- NICE TA1026: Tirzepatide for overweight and obesity
- Incretin-based drugs and gallbladder / biliary disease (The Lancet)
Final word for UK research customers
A retatrutide 40mg pen is a pre-filled, ready-to-use research peptide with 40mg in the tank — roughly 240 clicks, enough for many small doses or a handful of big ones. The pen format saves you the mixing and the maths. The one thing that really matters is being able to verify what is inside — and that is exactly what MyReta solves: every pen ships with a batch-specific third-party COA, a tamper-evident hologram and a unique batch code you can check the moment it arrives. So check the concentration, COA, batch details and storage before you order — or simply start with a 40mg pen that already ticks every box.
For research use only. Not for human consumption. Retatrutide is not licensed for human use in the UK.
