Reconstitution: doing the maths together

Who this is for, and what we will not do

This page is for people who are already holding a vial of powder and need to understand the numbers. That includes members with a legitimately compounded prescription from a regulated pharmacy, and members who are on a route with no prescription at all. We know both are here.

Three boundaries, stated once. We will not tell you a dose — not a starting one, not an increase, not "what most people use". Dose comes from a prescriber, and if you do not have one, that absence is a risk we cannot arithmetic away. We will not tell you where to buy anything; read sourcing outside a prescription for our honest position. And we will not pretend the calculation is the hard part of this route, because it is not — the hard part is that nobody has verified what is in the vial.

What we will do is walk the maths slowly enough that you can check your own working, because the errors here are not subtle-but-survivable. They are tenfold errors, and a tenfold error with these drugs means days of relentless vomiting and, for some people, a hospital bed. Sunil, who runs our research-reading circle and has an unfashionable enthusiasm for units, wrote most of what follows.

The three numbers, and the one relationship between them

Everything rests on three quantities. The mass of drug in the vial, stated in milligrammes. The volume of bacteriostatic water you add, in millilitres. And the concentration that results, in milligrammes per millilitre.

The relationship is division and nothing more:

concentration = mass in the vial ÷ volume of water added.

A vial containing 5 mg, reconstituted with 2 mL of water, gives 5 ÷ 2 = 2.5 mg per mL. The same 5 mg vial with 1 mL of water gives 5 mg per mL — twice as strong, so every measured volume delivers twice as much. The powder does not change. Only your water does.

Then, to find the volume that contains a given amount of drug, you divide again:

volume to draw = amount wanted ÷ concentration.

At 2.5 mg per mL, an amount of 0.25 mg sits in 0.25 ÷ 2.5 = 0.1 mL. At 5 mg per mL, that same amount sits in 0.05 mL. Two different volumes for the same quantity of medicine, purely because the water was different. This is why a chart from a forum post is worthless unless the vial size and the water volume match yours exactly.

Units are a volume, not a medicine

Now the part that hurts people. Insulin syringes are marked in units, and everyone reads that word as though it means a dose. It does not. On a standard U-100 syringe, the whole barrel of 100 units is one millilitre. So one unit is 0.01 mL, full stop, regardless of what liquid you put in it.

Carrying the earlier example forward: at 2.5 mg per mL, one unit — 0.01 mL — contains 0.025 mg. That is 25 microgrammes, since one milligramme is one thousand microgrammes. Ten units contain 0.25 mg. The syringe knows nothing about your drug and never has.

Two conversions to have in your head permanently. One millilitre is one hundred units on a U-100 syringe. One milligramme is one thousand microgrammes. Nearly every catastrophic error in this area is a failure of one of those two lines: somebody reads "units" as milligrammes, or slides a decimal point between mg and mcg.

Sunil's habit, which we recommend: write every quantity twice, once in mg and once in mcg, and once in mL and once in units. If the four numbers do not agree with each other, you have found your mistake before it found you.

The errors that have actually happened

These are not hypothetical. Regulators in the US have issued repeated alerts about dosing errors with compounded semaglutide, and poison centres logged a sharp rise in calls as vials spread. The recurring patterns:

• Units read as milligrammes. Somebody told to take a quantity in milligrammes draws that many units, or vice versa. This is the tenfold-and-worse error.
• Milligrammes read as microgrammes. A thousandfold error on paper, usually caught by disbelief, occasionally not.
• A chart for the wrong vial. The dosing table someone posted assumed a different vial mass or a different water volume. The numbers look authoritative and are simply not about your vial.
• Recalculating nothing after a change. The next vial is a different size, or you used a different volume of water, and the old routine carries on unchanged.
• Reading the wrong syringe. Not every insulin syringe is U-100, and a syringe from a different market may be marked differently. Check what is printed on the barrel.
• The unlabelled vial. Two vials in a fridge at different concentrations, and no marker pen anywhere in the house.

Tam described drawing up in a dark kitchen at midnight, realising she could not remember which vial was which, and throwing both away. That was the right call and it cost her money, which is exactly why the labelling step below is not optional.

Mixing it, physically

The mechanical part is less error-prone than the arithmetic, but it has its own rules and most of them are about not destroying the peptide.

Clean hands, clean surface, everything laid out. Swab the rubber stopper of both the powder vial and the water with alcohol and let it dry properly. Draw your measured volume of bacteriostatic water — measured, not estimated, because that number is half your concentration calculation.

Add the water slowly, aimed at the glass wall of the vial so that it runs down rather than jetting onto the powder. Peptides are damaged by force. Then leave it alone. Do not shake it. Swirl gently, or simply let it sit for a few minutes; most powders dissolve on their own and the liquid should end up clear and free of particles.

If it does not fully dissolve, or if there is anything visible floating in it once it has, do not use it.

Bacteriostatic water contains a preservative — usually benzyl alcohol — which is what allows a vial to be entered more than once without bacteria multiplying. Sterile water has no preservative and is a single-entry proposition. Benzyl alcohol is also not appropriate for everybody, and this is a genuine question for a pharmacist rather than a detail to skip past.

Label the vial the moment it is mixed: what it is, the concentration you calculated, and the date. Permanent marker, on the vial, not on a note.

Check your working with somebody else

Hospitals require two people to independently check high-risk calculations. They do not do that because pharmacists are bad at maths; they do it because everybody is bad at maths at eleven at night, and because a person who has just made an error is the worst possible person to spot it.

So: do the calculation, write it down, and then have a second person redo it from the original numbers rather than checking your answer. A partner, a friend, a buddy from buddies, anyone who can divide. If you genuinely have nobody, use our reconstitution tool as the second opinion — put your own numbers in and see whether it agrees with what you got by hand. If the two disagree, stop and find out why. Do not split the difference.

Then write the result somewhere permanent: the vial label, and the same note where your injection dates live. Members who have run vials for years all converged on the same habit, which is that the arithmetic is done once per vial, in daylight, sober, and never again until the next vial.

Wren's rule from the practical circle stands: arithmetic after nine at night gets done twice or gets done in the morning.

The limits the maths cannot reach

Perfect arithmetic on an unverified vial gives you an accurately measured amount of something you cannot identify. That is worth sitting with.

For compounded material from a regulated pharmacy, the concentration on the label is at least made under a compounding standard with some oversight, though even there content uniformity and stability are far less established than for a licensed product. For research-use-only material, the label is a claim by the seller and nothing more. A certificate of analysis tells you something about a sample somebody submitted, which is not the same as the vial in your fridge — reading a COA line by line and what purity actually means go through this properly, and third-party testing covers who actually tests what.

There is also no published stability data for reconstituted compounded material. Any expiry rule you see quoted for it is somebody's convention, not a finding. Keep it cold, keep it labelled, and do not stretch a vial because it feels wasteful.

We would rather you had this page than not. We would also rather you had a prescriber. Both of those are true at once, and nobody in sourcing safely will make you feel small for being where you are.