Priming a pen and whether the primed volume is wasted

Priming a pen and whether the primed volume is wasted Writing it up because I had to work it out twice and would rather nobody else did.

I have spent a fortnight trying to pin priming a pen down and I want to set out where I have got to, because I suspect the honest answer is duller than the thread this will produce.

What I have: a consistent observation across a small number of cases, collected the same way each time. What I do not have: any controlled comparison, or any reason to think my cases are representative.

The specific question is whether the pattern survives once the obvious confounder is removed. I cannot remove it with what I have.

The opening post answers the question as asked. The question underneath it is different.

Air in a cartridge behaves differently from air in a syringe because the mechanism displaces a fixed volume regardless. That is what priming addresses.

Everything in the opening post holds. The case it does not cover is the one I have.

A pen that has been dropped may not be visibly damaged and may not deliver accurately afterwards. Manufacturers say to replace rather than to test, and there is no test a user can do.

Narrowing post #5, because the general version has more than one answer.

A pen that will not dial to the intended dose is usually telling you the cartridge does not contain enough for it. That is a designed behaviour rather than a fault.

Reusability: some pens are single-use (throw away after one injection). Others can be reused until empty. Confirm which type you have. Reusing a single-use pen is ineffective because the mechanism does not reset.

I would put a moderate confidence on that and no more.

Holding the pen in place for the stated count after the plunger stops is the manufacturer's answer to leakage at the site. It is on the instruction leaflet and it is the step most often skipped.

I checked the source rather than the summary, and they differ.

Answering the question post #5 raises rather than the one it answers.

Disposal of a used pen is a sharps question even where the needle is retracted, and local rules vary more than people expect.

I am not the right person to answer the follow-up to this.

Priming: most pens require priming before the first use (injecting a small amount to fill the needle and line). Not priming can result in no dose being delivered on the first injection.

That is the version I use. It may not be the version that is correct.

Storage instructions for a pen typically differ before and after first use, and the in-use period is the one people miss. It is on the carton.

Post #8 put the caveat in the right place and I want to underline it.

Pen confusion between compounds: if you are using multiple pens or alternating pens, confirming you have the correct pen before injection prevents administration errors. Check the label every time.

I have seen it go both ways, which is why I hedge.

Storage after first use: most pens can be stored at room temperature after first use, even if unrefrigerated, for a limited time (often 28 days). Check your specific pen's labelling.

Stating my assumptions rather than smuggling them in.

Nothing here is medical advice, and pen technique is the single easiest thing to have demonstrated in person rather than learned from a description.