How much bacteriostatic water? Reconstitution is one division

How do you work out mg/mL after reconstituting?

Concentration is mass over volume, and that is the whole of it:

concentration (mg/mL) = vial strength (mg) ÷ water added (mL)

Peptrend’s reconstitution calculator is that expression, written out:

var concentrationMgPerML: Double { waterML > 0 ? strengthMg / waterML : 0 }

Two inputs go in. The strength field asks for “the total mg in the vial, before any water”, with shortcut presets at 5, 10, 15, 30, 50, 100 and 1000 mg plus a free-entry field that remembers whatever you typed last time. The water field asks for “however much you actually added”, with presets from 0.5 mL to 5.0 mL. Those two grids alone span concentrations from 1 mg/mL to 2000 mg/mL.

One number comes out, printed to two decimal places as mg/mL, with the two inputs echoed back beside it so you can see what produced it. The results panel stays pinned to the bottom of the sheet while you scroll, because the thing you are changing your mind about is the water, and you want to watch the strength move while you do it.

Why do peptides come as a powder?

Lyophilisation is freeze-drying, and it exists to keep the water away from the peptide until the last possible moment. In aqueous solution a peptide degrades along three families of route — hydrolysis of the backbone, deamidation of asparagine and glutamine residues, and physical aggregation — all catalogued in a 2023 Pharmaceutics review of formulation strategies for peptides in solution. Take the water out and the reactions that need water stop happening. The price is that somebody has to put it back.

When a product ships with its own matched diluent, the label does the division for you. Glucagon for injection is a lyophilised powder: 1 mg in the vial, 1 mL of sterile water in, and the FDA label reports “approximately 1 mg of glucagon per mL” coming out.

Bacteriostatic water is water for injection with 0.9% — 9 mg/mL — benzyl alcohol added as a preservative. Its FDA label describes a multiple-dose container “from which repeated withdrawals may be made”, and states it is “indicated only for diluting or dissolving drugs for intravenous, intramuscular or subcutaneous injection, according to instructions of the manufacturer of the drug to be administered”. Repeated withdrawals from one container are a designed property of the preservative.

The same 6 mg vial, mixed four ways

The app’s logged data is four vials transcribed from a real log, described in-app as “Four 6mg vials from a real log, mixed 3.0mL down to 0.7mL”. Same strength every time. The water moved and the concentration followed it, landing on three distinct answers across the four mixes — two of those mixes both took 1.0 mL, so they came out identical:

6 ÷ 3.0 = 2.00 exactly. 6 ÷ 0.7 = 8.571428…, which the display rounds to 8.57. The rounding lives in the label; a saved vial stores the strength and the diluent volume as separate fields and recomputes the quotient every time it needs to show it, so nothing is ever remembered at two decimal places.

Between the loosest and the tightest of those mixes there is a factor of 4.29 in strength, produced entirely by the water. Both vials held 6 mg.

Does adding more water mean less peptide?

No. The water sets how much peptide sits in each millilitre, and it does not add or remove a milligram: 6 mg dissolved in 3.0 mL is 6 mg, and 6 mg dissolved in 0.7 mL is 6 mg. Mass is the constant that survives the mix, which makes concentration and volume inversely proportional:

C × V = 6 mg

Halve the water and you double the strength. That hyperbola also means the arithmetic is least forgiving where the volumes are smallest. Slip by 0.05 mL on a 3.0 mL mix and the concentration is 1.6% off. The same 0.05 mL on a 0.7 mL mix puts it 6.7% off, roughly four times the error from an identical wobble of the plunger.

None of which the app can see. It has no idea what actually went into the vial; it takes the number you tell it and divides. That is why the calculator asks for “however much you actually added” rather than however much you meant to, and why its closing line is a suggestion to write the result down: “That’s what this vial holds once mixed. Write it on the vial so you don’t have to remember.”

Does Peptrend say how many units to draw?

No. Peptrend computes concentration and refuses to go one step further. It will not convert a dose in mg into a volume to draw, and it will not turn that volume into insulin-syringe units. Both functions used to exist. Both were deleted — volumeML(forDoseMg:) and insulinUnits(forDoseMg:) are gone from the vial model, and the comment left in their place explains why.

The reason is Apple’s App Review guideline 1.4.2: “Drug dosage calculators must come from the drug manufacturer, a hospital, university, health insurance company, pharmacy or other approved entity, or receive approval by the FDA or one of its international counterparts.” A personal tracking app is none of those things. The screen says so in the app’s own words: “A record of what you mixed, nothing more. Peptrend does not work out what to draw, what to take, or whether to.”

So the boundary is drawn at the division. Milligrams in, millilitres in, mg/mL out, saved with a date. Everything downstream of that number belongs to you and whoever you take medical advice from. Why a calculator should refuse argues the case at length.

What does the app save when you mix a vial?

A vial record holds the medication name, the strength in mg, the diluent in mL, the date and time you mixed it, an optional label, and an active flag. Saving is a toggle, on by default — you can run the division and keep nothing at all.

Save it and the record earns its place later. Each dose can be tagged with the vial it came from, the vial history card counts how many shots were drawn from each one, and deleting a vial untags those doses rather than taking them with it. The vial mix chart draws one bar per vial: bar height is the water you added, bar colour is the concentration that resulted, mapped onto the same gradient that shades doses elsewhere in the app. Four 6 mg vials at 2.00, 6.00, 8.57 and 6.00 mg/mL read as three heights in three colours, with the two 1.0 mL mixes drawing the same bar twice.

The calculator and the mix chart are Pro. The vial history card is not — a record you already saved stays visible whatever you are paying. All of it sits in your own private CloudKit database with no account and no server of the app’s own behind it, which the privacy page sets out in full. How long a mixed vial is good for follows one from the mix to the last dose drawn out of it.

Why is every dose in milligrams?

Every dose in the app is recorded in milligrams. There is no IU picker and no mcg option; volume is always mL and concentration always mg/mL, and the weight side of the app is the only place a unit choice exists at all. That consistency is what lets a logged dose feed anything else.

The mg figure is what the level model consumes: doses in milligrams, a published half-life, a Tmax, and a one-compartment curve that reports a decayed milligram amount rather than any blood concentration. Reading the absorption curve covers what that shape does and does not mean, and what to log with every dose covers the fields that make the rest of it work.

The concentration on the vial never enters those calculations. It is a property of the mix, useful at the syringe and on the label, and the app treats it that way — one number, two decimal places, stored with a date.