What is proven – and what isn’t
For approved medicines, the in-use shelf life is stated in the prescribing information (SmPC) because it has been tested. Examples: according to the manufacturer, an opened Mounjaro KwikPen may be used for up to 30 days, an Ozempic pen for up to 6 weeks. After that it is thrown away – even if there is still solution left in it.
No such data exists for research peptides. Nobody has tested how quickly the active substance breaks down in solution or how long it stays sterile. Anyone quoting a shelf life of several months is guessing.
Two risks: loss of potency and germs
- Loss of potency: Peptides break down in solution over time, faster with heat and light. The solution often looks unchanged.
- Germs: Every puncture through the rubber stopper is a risk. The preservative in bacteriostatic water (benzyl alcohol) inhibits bacteria, but not indefinitely – hence the usual limit of 28 days after opening.
How to store it properly
- Keep reconstituted solutions and opened pens in the fridge at 2–8 °C, not in the door, and do not freeze
- Protect from light – original packaging or a dark box
- Disinfect the rubber stopper before every puncture, always use a new needle
- Note the mixing date – e.g. with the label from our Tracker
- Throw it away if it is cloudy, has flakes or is discoloured – and when in doubt
When to see a doctor
- Redness, swelling, warmth or pus at the injection site
- Fever or chills after an injection
- The effect is suddenly much weaker or stronger than usual
In an emergency: 112.
FAQ
How long does reconstituted tirzepatide keep?
For unapproved products there is no verified data. According to the manufacturer, the approved Mounjaro KwikPen can be used for up to 30 days after first use – much shorter than is often claimed.
How long does bacteriostatic water keep after opening?
Manufacturers usually state 28 days after the first puncture.
Can I freeze reconstituted peptides?
That is not recommended. Freezing and thawing can damage peptides, and there is no data on it.
Sources
This article does not replace medical advice. It gives no dosing recommendations. As of September 2026.