I reconstituted a 10mg vial with 2ml of bacteriostatic water and I want someone to check my unit arithmetic before I draw anything, because I keep getting two different answers.
What I actually want to know is how much material I am losing to dead space, and whether that explains why a 10mg vial gives me nine usable draws rather than ten.
I have searched first, so if this is covered somewhere point me at it and I will read it.
Answering the narrow version, because the broad one does not have a single answer. Dead space is the answer to the missing dose. A fixed-needle insulin syringe holds a few microlitres in the hub and needle after the plunger bottoms out, and on small draws that is a measurable percentage of every dose. Across ten draws it adds up to most of an eleventh, which is exactly the "nine draws from a ten-dose vial" complaint. Luer-lock syringes are worse; low-dead-space fixed-needle designs are better.
That is the short version; the long version is somebody else's post.
InsuranceTom said:Dead space is the answer to the missing dose.
InsuranceTom has the substance of this right. The condition it depends on is worth stating. The arithmetic is right and one caveat matters: a U-40 syringe changes the markings, not the volumes. If someone hands you U-40 and you read it as U-100 you will be 2.5 times out, and that error has a direction — it is always an overdose.
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Shop Reference StandardsLarryQC_SD said:I reconstituted a 10mg vial with 2ml of bacteriostatic water and I want someone to check my unit arithmetic before I draw anything, because I keep…
This matches mine closely enough to be worth saying so. Do it in two steps and it stops being confusing. First concentration: 10mg into 2ml is 5mg/ml. Then volume: a 0.5mg dose is 0.5 ÷ 5 = 0.1ml. Then units, and this is where people go wrong — a U-100 syringe is graduated in hundredths of a millilitre, so 0.1ml is 10 units. The word "units" has nothing to do with milligrams; it is a volume marking that exists because insulin happens to come at 100 units per ml.
Clinical perspective, offered as context rather than as advice.
Ongoing cost tracking for dosing arithmetic — transparency for the community:
Monthly medication cost: $146
Supplies (syringes, BAC water, alcohol swabs): ~$9/month
Janoshik testing: ~$11/month (amortized quarterly)
Lab work: ~$56/month (amortized quarterly)
Total: ~$216/month
Compare to brand Wegovy at $1336/month without insurance. Compounded is roughly 85% cheaper for what I have found to be clinically equivalent medication.