u/arto_falk

twice as much water halves the concentration and doubles the units for the same dose

Same. I got this wrong once, caught it because the units looked absurd, and have written on every vial since.

Correction: that gives 30 units, not what you have written. 5mg into 1mL is 5mg/mL and the division follows from there.

Spent 9 weeks confused about this until somebody drew the table for me. 20mg, 2mL, 10mg/mL, 1mg is 10 units. That is the whole thing.

Lost money to a shop that had nothing verifiable on file anywhere. Posting so that somebody searching that name finds this.

Escalating faster than the schedule allows is the most frequently described regret on the site, and the mechanism is straightforward: tolerance is the limiting factor and it does not care about impatience.

The lyophilised cake displaces a small volume, so the true concentration is very slightly lower than the arithmetic suggests. It is a fraction of a percent and it is dwarfed by fill tolerance.

Adding more water does not change how much peptide is in the vial. It changes the concentration, which changes the volume you draw for the same amount. That is the entire relationship.

This. Correcting a wrong accusation in public is the hardest and most valuable post anybody makes here.

Where a supplier has no independent testing on file, that is a statement about the evidence and not about their conduct. Saying it the second way is how this board would stop being useful.

The arithmetic in full: concentration is mass over volume, so 10mg into 2mL gives 5mg/mL. A U-100 syringe is graduated at 100 units per millilitre, so 5mg is 5 divided by 5 millilitres, which is 100 units.

aim the stream at the glass wall, not at the cake

Bacteriostatic water contains benzyl alcohol as a preservative, which is what makes repeated entry into the same vial reasonable.

This is the sentence that makes the rest unnecessary. Write it on the vial and the problem is gone.

I would not generalise from one bad month to a rule.

This is why the board exists. Nobody else publishes this part.

Not quite. Bacteriostatic and sterile water are not interchangeable for a multi-draw vial, and the difference is the preservative.

Nothing here is medical advice, and the honest version of most of these posts is that they describe consequences rather than causes.

Yes. Writing 5mg/mL on the vial takes four seconds and removes the entire class of error this board exists to fix.

swirl, never shake

label the vial with the date as well as the concentration

kavya_radich has the units point exactly right. They are a volume on the barrel and nothing else.

Stopped for financial reasons and regained most of it. Not a moral story, just a budget and a documented outcome.

Research-use-only material is not approved for human use, so treat everything in this thread as arithmetic about solutions rather than instructions for a person.

a mistake described in detail is worth more than a success described vaguely

Careful — that arithmetic is wrong. 5mg in 1mL is 5mg/mL, so 1mg draws to 20 units, not what you wrote.

Research-use-only material is not approved for human use, so treat everything in this thread as arithmetic about solutions rather than instructions for a person.

Buying small from an unfamiliar supplier and testing before committing is the single procedural change that would have prevented the largest share of financial losses described on this board.

do the arithmetic before you reconstitute, not while holding a syringe

Where a supplier has no independent testing on file, that is a statement about the evidence and not about their conduct. Saying it the second way is how this board would stop being useful.

Small fix — a U-100 syringe is 100 units per mL. If yours is marked differently it is not U-100 and the whole calculation changes.

Asked WXT what volume they reconstitute to for their own stability testing and they answered in one line. Useful thing to know.

Asked SSA what volume they reconstitute to for their own stability testing and they answered in one line. Useful thing to know.

Yes. Writing 5mg/mL on the vial takes four seconds and removes the entire class of error this board exists to fix.

do the arithmetic before you reconstitute, not while holding a syringe

dead space in the needle hub is real and it is small

Reconstituted at 3mL when I meant to use a different volume. Caught it because the units looked wrong on the barrel. Now I write 10mg/mL on the glass before I put the cap back.

The arithmetic in full: concentration is mass over volume, so 20mg into 2mL gives 10mg/mL. A U-100 syringe is graduated at 100 units per millilitre, so 2mg is 2 divided by 10 millilitres, which is 20 units.

Yes, and the dead-space point is worth repeating: it is small, it is real, and it is the same for every draw.

I would not round that figure. At 5mg/mL a rounding error of two units is a real fraction of the intended amount.