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Completing a dose with a second syringe, and the dead-volume arithmetic

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f.piresTL225 Jan 2026#1

Posting this under the heading it deserves: Completing a dose with a second syringe, and the dead-volume arithmetic Everything below is what sits behind that.

I have read the maintained page on this and I still have a gap, so I am asking rather than guessing.

Context: tirzepatide, 19 weeks in, currently at a dose I reached by the standard four-week steps. Everything below is my own record rather than anything a clinician told me.

The specific question is the one in the title. What I have already checked: the labelling summary on the relevant documentation page, the two most-linked topics in this subcategory, and my own notes from the last 10 weeks. What I could not find is whether the answer changes at higher doses or whether it is the same arithmetic throughout.

If the answer is "it depends", I would rather know what it depends on than be given a number.

33 likes 6mo

r.marsdenTL3Regular31 Jan 2026#2

Community wiki post. Any member at trust level 3 or above can edit this post; every edit is recorded. Last edited by r.venkatesan on 14 Feb 2026.

16 Apr 2026 — coldchain_liu: Removed a claim that the cited source did not support.
14 Feb 2026 — r.venkatesan: Replaced an unsourced figure with the published one and cited it.

Editors: coldchain_liu, r.venkatesan

My understanding of completing a dose is a few years old and may have been superseded. If it has been, I would genuinely like to know rather than keep repeating it.

0 likes 6mo

n.serranoTL25 Feb 2026#3

r.marsden, post #2: My understanding of completing a dose is a few years old and may have been superseded. If it has been, I would genuinely like to know rather than keep repeating it. Go to post

Completing a dose has a well-known answer and a correct answer, and the interesting work is establishing that they are the same. Nobody has done that here yet.

1 like in reply to #2 6mo

LeitermanTL3Regular9 Feb 2026 · edited#4

Nothing to add, except that this is the answer I would give if asked.

7 likes 6mo

m.ramosTL212 Feb 2026#5

If you have to inject at a different time of day than usual, that is a much smaller perturbation than it feels for a weekly compound with a week-long half-life.

25 likes 5mo

sterile_tableTL3Regular16 Feb 2026#6

A new needle for each injection is not fussiness. A needle blunts on first use and a blunted needle is the reason the second injection hurts more than the first.

I would put the burden of proof on the interesting explanation, not the dull one.

0 likes 5mo

y.eriksenTL219 Feb 2026#7

Leiterman, post #4: Nothing to add, except that this is the answer I would give if asked. Go to post

Building on post #5 rather than restating it.

If you have to inject at a different time of day than usual, that is a much smaller perturbation than it feels for a weekly compound with a week-long half-life.

The confident version of this sentence would be wrong, so here is the hedged one.

3 likes in reply to #4 5mo

PSundbergTL2Member22 Feb 2026#8

Post #6 put the caveat in the right place and I want to underline it.

Something worth flagging about completing a dose: the strongest-sounding claims in this thread are the ones with no source attached, which is the usual pattern and not a coincidence.

11 likes 5mo

a.adeyemiTL225 Feb 2026 · edited#9

Leave real distance between consecutive sites rather than moving a centimetre. Repeatedly injecting one area changes the local tissue over time and eventually changes absorption there.

The reasoning is more useful than the number, which is why I have shown it.

33 likes 5mo

revision_historyTL3Wiki editor1 Mar 2026#10

a.adeyemi, post #9: Leave real distance between consecutive sites rather than moving a centimetre. Repeatedly injecting one area changes the local tissue over time and eventually changes absorption there. The reasoning is more useful than the number, which is why I have shown it. Go to post

Everything in post #6 holds. The case it does not cover is the one I have.

The bit of completing a dose that nobody enjoys is that the answer changes depending on what you are trying to decide with it. Say what the decision is and the thread will converge.

0 likes in reply to #9 5mo

PSundbergTL2Member3 Mar 2026#11

sterile_table, post #6: A new needle for each injection is not fussiness. A needle blunts on first use and a blunted needle is the reason the second injection hurts more than the first. I would put the burden of proof on the interesting explanation, not the dull one. Go to post

Post #9 put the caveat in the right place and I want to underline it.

What I would want before treating completing a dose as settled: the method, the sample, and whether anyone tried to find the opposite result. Two of the three are usually missing.

17 likes in reply to #6 5mo

y.eriksenTL26 Mar 2026#12

If you have to inject at a different time of day than usual, that is a much smaller perturbation than it feels for a weekly compound with a week-long half-life.

7 likes 5mo

sterile_tableTL39 Mar 2026#13

Collapsed as off-topic by two members at trust level 3 or above

Completing a dose is well covered in the tag pages, and the older discussions are better than the recent ones because they were argued out properly. Worth twenty minutes before adding to this one.

m.ramosTL212 Mar 2026 · edited#14

y.eriksen, post #12: If you have to inject at a different time of day than usual, that is a much smaller perturbation than it feels for a weekly compound with a week-long half-life. Go to post

I will take the caveat as seriously as the claim, which is the point of putting it there.

0 likes in reply to #12 5mo

curious_readerTL1Member15 Mar 2026#15

On completing a dose: the maintained page in the documentation commons covers the general case with citations and a review date, which is more reliable than any reply here including this one.

23 likes 4mo

j.ivaturiTL217 Mar 2026#16

Nothing in this subcategory is medical advice and none of it is a substitute for a demonstration from somebody qualified to give one, which most prescribers will do without being asked twice.

11 likes 4mo

revision_historyTL3Wiki editor20 Mar 2026#17

Everything in post #15 holds. The case it does not cover is the one I have.

Bleeding at the injection site: normal and usually minimal. A little blood at the needle site after withdrawal is not a reason to assume you lost a significant dose. The needle passed through tissue and there is a small amount of bleeding in the tract.

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

3 likes 4mo

a.adeyemiTL223 Mar 2026 · edited#18

revision_history, post #17: Everything in post #15 holds. The case it does not cover is the one I have. Bleeding at the injection site: normal and usually minimal. A little blood at the needle site after withdrawal is not a reason to assume you lost a significant dose. The needle passed through tissue and there is a small amount of bleeding in the tract. That is… Go to post

Narrowing post #17, because the general version has more than one answer.

Answering the completing a dose question as asked, then the question I think is meant. As asked: yes, with the qualification below. As meant: it depends on how the first measurement was taken.

0 likes in reply to #17 4mo

j.habermannTL3Regular25 Mar 2026#19

Leak-back after withdrawal: a small amount of liquid on the skin after withdrawal is ordinary. Whether you lost a "meaningful" dose depends on concentration and your target dose, but some leak-back is universal and does not need to be treated as an error.

31 likes 4mo

k.okaforTL228 Mar 2026#20

This follows post #17 rather than contradicting it.

Where I have landed on completing a dose, having got it wrong once in public: the direction is clear, the magnitude is not, and anyone quoting a precise magnitude has borrowed it from somewhere that did not measure it.

16 likes 4mo

c.inglethorpeTL3Regular30 Mar 2026#21

Post #18 is the version of this I will quote in future. One addition.

Completing a dose is one of those subjects where the general answer and the answer for a specific case diverge, and the thread will go in circles until someone says which one is being asked for.

5 likes 4mo

f.chowdhuryTL22 Apr 2026#22

c.inglethorpe, post #21: Post #18 is the version of this I will quote in future. One addition. Completing a dose is one of those subjects where the general answer and the answer for a specific case diverge, and the thread will go in circles until someone says which one is being asked for. Go to post

Where I part company with post #20, and it is a narrow parting.

If you have to inject at a different time of day than usual, that is a much smaller perturbation than it feels for a weekly compound with a week-long half-life.

Worth checking against a second source before it gets quoted onward.

15 likes in reply to #21 4mo

CSagredoTL3Regular4 Apr 2026 · edited#23

Air in the syringe is a much smaller problem subcutaneously than the anxiety about it suggests, and it is still worth expelling because the bubble occupies volume you meant to be liquid.

0 likes 4mo

r.molnarTL27 Apr 2026#24

Reading back through, this was answered upthread and I missed it. My fault.

0 likes 4mo

m.eriksenTL29 Apr 2026#25

Building on post #22 rather than restating it.

A genuinely late dose: the labelling generally says that if the next dose is more than a couple of days away, take it as soon as you notice. If the next dose is close, skip it and resume the schedule. The reasoning follows from the long half-life; you are perturbing a slowly moving average.

Second-hand, so weight it accordingly.

9 likes 4mo

m.ekstromTL212 Apr 2026#26

n.serrano, post #3: Completing a dose has a well-known answer and a correct answer, and the interesting work is establishing that they are the same. Nobody has done that here yet. Go to post

If you have to inject at a different time of day than usual, that is a much smaller perturbation than it feels for a weekly compound with a week-long half-life.

21 likes in reply to #3 4mo

ThibodeauTL3Regular14 Apr 2026#27

PSundberg, post #11: Post #9 put the caveat in the right place and I want to underline it. What I would want before treating completing a dose as settled: the method, the sample, and whether anyone tried to find the opposite result. Two of the three are usually missing. Go to post

I disagree with the framing of completing a dose above, and I think it is a substantive disagreement rather than a terminological one. Setting out why, so it can be checked.

The reasoning depends on an assumption that is doing a lot of work and is never stated. If the assumption holds, the conclusion follows. I do not think it holds generally.

0 likes in reply to #11 3mo

h.amankwahTL216 Apr 2026#28

Note the site alongside the dose. If a local reaction appears three weeks later you will want to know where the previous four injections went.

Scoping that to what I have actually seen rather than what I have read.

2 likes 3mo

IMainwaringTL3Regular19 Apr 2026#29

Thibodeau, post #27: I disagree with the framing of completing a dose above, and I think it is a substantive disagreement rather than a terminological one. Setting out why, so it can be checked. The reasoning depends on an assumption that is doing a lot of work and is never stated. If the assumption holds, the conclusion follows. I do not think it holds… Go to post

Worth separating completing a dose as a question about the compound from completing a dose as a question about the documentation. They get answered by different people and only one of them is answerable here.

1 like in reply to #27 3mo

l.lundgrenTL221 Apr 2026#30

6 likes 3mo

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