Cost per dose: the math for reconstituted vials

Step 1 — doses per vial

Doses per vial = vial strength (mg) ÷ your dose (mg).A 5 mg vial at a 0.5 mg dose → 10 doses. A 10 mg vial at 2.5 mg → 4 doses. Note what's not in this formula: the bacteriostatic water. Adding more water dilutes the solution (you draw more units per dose) but the total drug in the vial — and therefore the number of doses — doesn't move.

Step 2 — cost per dose

Cost per dose = vial price ÷ doses per vial.That $300, 5 mg vial at 0.5 mg per dose: $300 ÷ 10 = $30 per dose. The same vial at a 1 mg dose is $60 per dose — cost per dose scales directly with dose size, which is why the same vial can be cheap for one person and expensive for another.

Step 3 — cost per month

Cost per month = cost per dose × doses per month.For weekly dosing, a month is about 4.35 weeks (52 ÷ 12), so: $30 per dose × 4.35 ≈ $130/month. Using 4 flat makes months look cheaper than they are — over a year that shortcut hides ~a full vial of spend.

The gotcha: partial vials

Doses per vial rarely divide evenly. A 5 mg vial at 0.75 mg is 6 full doses with 0.5 mg stranded — you either waste it or your 7th dose is short. When comparing vial sizes, compare usable doses (round down), not the raw division. The calculator does this rounding for you and shows the exact per-dose cost from your own numbers.

Worked example, end to end

10 mg vial, $420, 2 mL of bac water, 1.25 mg dose: doses per vial = 10 ÷ 1.25 = 8 · cost per dose = 420 ÷ 8 =$52.50 · monthly (weekly dosing) = 52.50 × 4.35 ≈$228. The 2 mL of water never entered the math — it only determines that each dose is 25 units on a U-100 syringe.