What the "fifth dose" actually was
UK Mounjaro KwikPens are labelled for four weekly doses. The original UK pen (now called V1) contained 3.2 ml of fluid. Four doses use 4 × 0.6 ml = 2.4 ml. Leaving roughly 0.8 ml behind. That leftover was deliberate overfill: manufacturers include extra fluid to guarantee every labelled dose is full-strength even accounting for priming and mechanical tolerance.
Users noticed. Forums filled with methods to dial out or draw out a "free" fifth 60-click dose, effectively turning a four-week pen into five weeks. A 25% cost saving at a stroke. It became one of the most-discussed topics in UK GLP-1 communities.
Then the pen changed: V1 vs V2
| Legacy V1 pen | Current V2 pen | |
|---|---|---|
| Total fluid | 3.2 ml | 2.6 ml |
| Four labelled doses use | 2.4 ml | 2.4 ml |
| Leftover after 4 doses | ~0.8 ml (a full 5th dose) | ~0.2 ml (priming margin only) |
| Fifth dose possible? | Physically, yes (never licensed) | No. The fluid is not there |
The redesigned V2 pen holds 2.6 ml. Just the four doses plus the small priming margin the mechanism needs. The dial may not physically lock after dose four, but there is no longer enough fluid to extract a full fifth injection. Any content telling you otherwise is describing the old pen.
Check the fluid volume printed on your pen label and carton: 3.2 ml = V1, 2.6 ml = V2. If you somehow still hold a V1 pen, check the expiry date carefully before doing anything with it.
Why extracting leftovers was never a good idea
Even when the fifth dose physically existed, it sat firmly outside the licensed instructions. For reasons that are about your safety, not Eli Lilly's revenue:
- Dose accuracy isn't guaranteed. Overfill exists to protect the four labelled doses. Concentration in the residual fluid isn't quality-assured as a fifth dose, and partial draws produce partial, unpredictable doses.
- Contamination risk. Drawing fluid out with a syringe breaches the sealed pen system. The pen contains preservative for its intended use pattern. Not for being tapped like a vial over five weeks.
- Needle-stick and injection errors. DIY extraction methods circulating online involve techniques most people have never been trained in.
- Your prescriber can't supervise what they don't know. If a fifth dose causes side effects, your clinical team is flying blind about what you actually took.
Four doses per pen is the design, the licence, and the safe answer. If cost is the pressure (and we understand it often is) compare providers on cost per dose using our click calculator, and use a manually verified pharmacy list like Monj.co.uk rather than stretching pens beyond their design.
Legitimate ways to make Mounjaro cost less
- Stay at the lowest effective dose. There is no rule saying you must titrate to 15 mg. Many people maintain well at 5 mg or 10 mg, and lower-strength pens often cost less.
- Compare cost per dose, not per month. Pharmacy pricing varies enormously for the identical product. The calculator does this maths for you.
- Check NHS eligibility. If you meet the NICE TA1026 criteria (BMI ≥35 plus a weight-related condition, with lower thresholds for some ethnic backgrounds), ask your GP what is commissioned in your area as the rollout expands.
- Ask about prescriber-guided tapering. Some clinicians support click-counted dose adjustments between strengths. Done openly, with sterile technique and a plan, it is a different world from forum extraction guides.
Frequently asked questions
Can you still get a fifth dose from a Mounjaro pen? +
Not from the current pen. The redesigned V2 KwikPen contains 2.6 ml. Four doses (2.4 ml) plus only the priming margin. The fifth-dose method described online applied to the legacy 3.2 ml V1 pen, and was never part of the licensed instructions even then.
How do I know if my pen is V1 or V2? +
Check the volume printed on the pen and carton: 3.2 ml is the legacy V1, 2.6 ml is the current V2. Pharmacies have been supplying V2 pens for some time now.
Was the fifth dose dangerous? +
It carried real risks (inaccurate dosing, contamination from breaching the sealed pen, and injection errors) which is why it was never recommended by Eli Lilly, the MHRA or any clinical body, even when the fluid physically existed.
Why did Eli Lilly change the pen? +
The redesign aligns the fluid volume with the four labelled doses. Whether motivated by manufacturing efficiency, dose-integrity or both, the practical result is the same: the pen now contains what the label says, and little more.
What is the safest way to reduce Mounjaro costs? +
Compare pharmacies on cost per dose (our calculator does this), discuss staying at a lower effective dose with your prescriber, check NHS eligibility under NICE TA1026, and use manually verified pharmacy lists such as Monj.co.uk to find legitimate competitive pricing.
References & official sources
Show all 4 cited sources, with study authors +
- electronic medicines compendium. Mounjaro KwikPen SmPC & patient leaflet: www.medicines.org.uk/emc/search?q=mounjaro
- Eli Lilly UK. Mounjaro KwikPen storage, transport and travel guidance: medical.lilly.com/uk/products/answers/how-should-patients-store-the-mounjaro-tirzepatide-kwikpen-during-transport-and-travel-217602
- NICE TA1026. Tirzepatide for managing overweight and obesity: www.nice.org.uk/guidance/ta1026
- MHRA. Yellow Card reporting scheme: yellowcard.mhra.gov.uk/