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.

Practical question with the units stated, because I have seen how quickly these go wrong without them.

I have a 2 mg vial of tirzepatide and I am working to a 0.5 mg step. My syringes are U-100 insulin syringes, 0.3 mL barrel.

I can do the arithmetic and I have done it twice, getting the same answer both times, but I would like someone to check the reasoning rather than the number — specifically whether I have thought about the residual volume correctly, and whether the graduation I am landing on is one a person can actually read.

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

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.

Dose counters: pens show the remaining number of doses. When the counter reaches zero, the pen is empty and cannot deliver more. Noting the counter before injection prevents running out unexpectedly.

Worth separating two things that the opening post runs together.

Injection duration: most pens deliver the dose over several seconds. Holding the button down for the full duration ensures complete delivery. Releasing early can result in underdosing.

post #5 is right about the mechanism and I think understates the practical bit.

Off-label dosing with a pen: if you want to deliver a dose different from the pen's pre-set steps (e.g., halfway between two step doses), that is off-label use. Some pens allow it; others do not. Confirm before attempting.

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.

Thank you for the correction. I have edited my earlier post with a note rather than silently, so the thread still makes sense to read. The error was mine and it was the kind that comes from remembering a figure instead of looking it up.

On post #5 — agreed on the reasoning, with one qualification.

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.

For anyone arriving from a search: the marked solution above is the direct answer, and the replies underneath it add the caveats that make it safe to use.

Needle installation: some pens require a fresh needle for each injection; others have a single needle that stays attached. Check the design of your pen.

Worth separating two things that post #8 runs together.

Visibility of the dose: some pens allow you to see the insulin remaining; others do not. Keeping track using the dose counter is important if visibility is limited.

For anyone arriving from a search: the marked solution above is the direct answer, and the replies underneath it add the caveats that make it safe to use.

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.