You can just take less Mounjaro
tl;dr: you can subdivide Mounjaro pens to get less than the stated dose from them. This lets you e.g. buy a 15mg pen and instead get 5mg at a time out of it (so you’d get 12 doses instead of 4). This works out to be much cheaper than buying a pen of the correct dose, and cheaper than using grey market peptides. Use this calculator to figure out how much to use.
[This is not medical advice I am not your doctor etc etc]
(See also: “You’re not sick enough for this medicine.”)
Miracle weight-loss drug Mounjaro comes in fixed-dose pens and a fixed dose escalation schedule. You can 1) choose to ignore this schedule and just stay on a low dose, and 2) subdivide the pens to make them last longer and save a lot of money.
In the US these are often autoinjector pens; there’s no way to customise the dose as it just gives you what’s printed on the side. In other countries you get a KwikPen, which is more like an insulin injector.
You take off the lid, screw on a single-use needle, twist the dial on the end until the window shows “1”, jab yourself with the needle and slowly push down the plunger.
Technically you can “count clicks”. The dial clicks as you rotate it; a full dose is 60 clicks, so 30 clicks gives you half a dose, 15 a quarter, etc. But the pen will only deliver four doses regardless of size and then it locks you out!
There’s a trivial and safe way to get around this: use an insulin needle to draw the amount of liquid that you want, and then inject it directly. You can buy the highest-dose pen, which contains four 15mg doses (60mg total) and get e.g. 24 × 2.5mg doses from it. This means you can stretch out a single pen to last many months.
The cost savings are substantial – a 2.5mg pen costs £37.24 per dose, but a 15mg pen is only slightly more expensive. So if you’re using a 15mg pen but only taking 2.5mg at a time, it works out at £12.46 per dose. This is less even than grey market Chinese peptides, which often run $100 for 10mg.11. Based on prices listed at ASDA Online Doctor as at publication. Example per milligram prices: 2.5 mg: £148.97 ÷ 10 mg = £14.90/mg 5 mg: £188.97 ÷ 20 mg = £9.45/mg 7.5 mg: £248.97 ÷ 30 mg = £8.30/mg 10 mg: £278.97 ÷ 40 mg = £6.97/mg 12.5 mg: £288.97 ÷ 50 mg = £5.78/mg 15 mg: £298.97 ÷ 60 mg = £4.98/mg So even using a 5mg pen for 2.5mg doses is a substantial cost savings, especially added up over a long period of time – most patients will take Mounjaro for months or years to get to their goal weight, and often will stay on a 2.5mg a week maintenance dose indefinitely.
Don’t the pens expire?
Yes, and this is where this trick runs counter to the manufacturer’s advice.
Once you start using the pen, it’s been exposed to the air and to any bacteria on the needle. There is a risk of bacterial growth in the liquid in the pen – the typical guidance is to not use an opened multidose vial beyond 30 days – but this is probably overstated.
The tirzepatide solution in the pen contains benzoyl alcohol which inhibits bacterial growth, although not forever. In the case of e.g. multi-dose insulin vials, using them well beyond 28 days shows negligible contamination in practice – one study found only trace skin flora after 53 days of use, another found the preservatives actively killed deliberately introduced bacteria through day 50, and a third found zero contamination across six months of twice-daily use22. In a study of 69 multi-dose insulin vials used by patients for an average of 53 days, only 8 showed any bacterial contamination at all — just 1 colony-forming unit per millilitre of common skin flora (S. epidermidis and P. acnes), with no endotoxin detected. Critically, when vials were deliberately inoculated with S. aureus and P. aeruginosa and kept at room temperature, they were sterile within 48 hours — and this antibacterial effect was maintained through serial re-contamination at days 17, 30, and 50. A more recent study went further: refrigerated multi-dose insulin vials aspirated twice daily with a new syringe for six months showed no microbial contamination at any point during the study period. A human retrospective study of insulin glargine used up to 74 days beyond the recommended duration found no injection site infections.. Anecdotally people have used the pens for many months after they’re first used without issue.
But the medical guideline is to use the pens for no longer than 30 days after opening. It’s up to you to decide your risk appetite here. Aaron Kaufman wrote an excellent post on how long you can use peptides after reconstitution (which in our case is just after the first time you’ve used the pen):
I’ve concluded that the 28-day limit appears to be conservative regulatory boilerplate mostly divorced from any specific scientific reasoning. … Based on the considerations above, I personally throw out refrigerated reconstituted peptide vials at about the 4-month mark, which is almost totally arbitrary. I make no judgement on what anyone else should do.
The peptide itself is stable in the fridge for many months, up to the expiry date on the pen.
Staying on low doses
The typical dose escalation schedule is to start on 2.5mg weekly for four doses, and then increase stepwise every four weeks up to 15mg. But plenty of users don’t need to go to the higher doses, with many staying at 2.5mg. There’s no good reason to increase your dose if you’re seeing satisfactory results at lower doses.
You can even take 1.25mg, or half the starting dose, if you’re concerned about side-effects when you start.
Guide to getting smaller doses
You’ll need an insulin needle and a Mounjaro KwikPen. You can essentially follow any guide to injecting from a multi-dose vial, you’re just using the pen itself as a vial.
Wipe the rubber septum with an alcohol wipe, shown here:
Uncap the needle and insert it through the septum.
Draw up the amount of liquid you want.
Remove the needle from the pen and administer it with correct technique.