Peptide Reconstitution and Dose Logging: A Working Reference

Marker: PTREF-ANCH-7Q4K2

Most dosing mistakes with reconstituted peptides are not pharmacology mistakes. They are arithmetic mistakes made at the kitchen table with a syringe that is marked in units rather than milligrams. This page walks the arithmetic in both directions and lists what a dose log should actually record.

The one equation that matters

concentration = total mg in the vial / mL of bacteriostatic water added

A 5 mg vial reconstituted with 1 mL of bacteriostatic water gives 5 mg/mL. The same 5 mg vial reconstituted with 2 mL gives 2.5 mg/mL. The vial did not change. The number you draw did.

A U-100 insulin syringe is marked 1 to 100, where 100 units equals 1 mL. So one unit is 0.01 mL.

Forward (mg to units): units = (dose in mg / concentration in mg/mL) x 100

At 5 mg/mL, a 0.25 mg dose is (0.25 / 5) x 100 = 5 units. At 2.5 mg/mL, that same 0.25 mg dose is (0.25 / 2.5) x 100 = 10 units.

This is the trap. An identical prescribed dose is a different number on the barrel depending only on how much water went into the vial. People who switch vial sizes or suppliers and keep drawing "the same number of ticks" are the ones who accidentally double or halve themselves.

Reverse (units to mg): mg = (units / 100) x concentration in mg/mL

You drew 12 units from a 10 mg vial reconstituted with 2 mL (5 mg/mL). That is (12 / 100) x 5 = 0.6 mg. Running this check after the fact catches transcription errors, and it is the calculation a peptide reconstitution calculator should show you rather than hide.

Choosing a reconstitution volume

There is no clinically correct volume. There is only a volume that makes your dose land on a readable mark. Pick the water volume so your typical dose falls between roughly 10 and 40 units on a U-100 barrel. Below about 5 units the meniscus error is a large fraction of the dose; above about 60 units you are drawing more liquid than a subcutaneous site takes comfortably.

Add the water slowly against the glass wall, do not jet it into the powder, and do not shake. Swirl and wait. Once reconstituted, storage is refrigerated and the working life is finite; label the vial with the date and the resulting concentration, not just the mg.

Titration steps

Titration exists to let gastrointestinal side effects settle before the next increase, not to reach a target faster. The common pattern is a fixed low starting dose held for four weeks, then a step up, then another four weeks. If a step produces nausea that does not settle within the interval, the usual move is to hold at the current step for another cycle rather than proceed. Steps down are legitimate. Any actual dose decision belongs to your prescriber.

Half-lives and why the schedule is weekly

Semaglutide has a terminal half-life of roughly 7 days, which is why a once-weekly injection produces relatively flat plasma levels and why steady state takes 4 to 5 weeks to arrive. Tirzepatide sits around 5 days, also weekly-dosed. The practical consequence: a change you make today is not fully expressed for a month. Judging a dose after one week is judging noise. It also means a missed dose is not an emergency, but doubling up to compensate is a bad idea, since the drug from the missed week has not fully cleared.

Injection site rotation

Abdomen (avoiding a two-inch ring around the navel), outer thigh, and back of the upper arm are the usual subcutaneous sites. Rotate between regions week to week and between spots within a region, keeping roughly an inch from the previous puncture. Repeated injection into one small patch produces lipohypertrophy, and absorption from a lumpy site is erratic, which reads as a dose that "stopped working." A GLP-1 dose tracker that stores the site alongside the dose makes the rotation visible instead of remembered.

What a dose log should record

Minimum useful fields per entry:

Date and time of injection
Compound and vial lot
Vial strength in mg and the water volume used, or the derived mg/mL
Units drawn AND the computed mg (record both, so a later mismatch is detectable)
Injection site
Weight, and any side effect worth noting

Recording units alone is the most common logging failure. Units are meaningless once the vial changes. Recording mg alone loses the ability to audit the draw. Record both.

The Peptide Tracker & Calculator is a free iPhone app that does this arithmetic and keeps the log on the phone, which is the shape this problem wants: the calculation happens where the syringe is.

A note on scope

Nothing here is dosing advice. Concentrations, titration schedules, and whether a compound is appropriate at all are decisions for a licensed prescriber who knows your history. This page covers the arithmetic and the record-keeping, which are the parts people most often get wrong on their own.