How Reko works, and the science behind it

1. The estimated level

When you log a dose, Reko draws a curve of roughly how much of that compound is in your body over the following days and weeks. The curve comes from two things only: the doses you logged, and the compound's published half-lives.

What a half-life is

A half-life is the time it takes for the amount of something in the body to fall by half. Two matter here. The absorption half-life describes how quickly an injected dose moves from the injection site into circulation. The elimination half-life describes how quickly the body clears it afterwards.

Why the curve starts at zero

Right after an injection, none of the dose has been absorbed yet. So the curve begins at zero, rises to a peak, then falls. A model that treats the full dose as present the instant you inject would jump straight to the top. Real absorption from under the skin takes hours to days, so Reko includes the absorption half-life that such models leave out.

The model

Reko uses a one-compartment model with first-order absorption. In words: the body is treated as a single pool, and both absorption and elimination follow a fixed fraction per unit of time. For one dose, the amount in the body at time t is:

A(t) = (F · D · ka) / (ka − ke) · (e−ke·t − e−ka·t)

• D is the dose you logged.
• F is the fraction of the dose that reaches circulation (bioavailability).
• ka and ke are the absorption and elimination rates, each worked out from its half-life as ln(2) divided by the half-life.

When the two rates are almost equal, the formula divides by nearly zero, so Reko switches to its mathematical limit, which joins smoothly with the general form.

Several doses add up

The model is linear, so the amount from many doses is the sum of each dose's own curve. That is how the curve shows build-up across weekly injections, or across a titration where the dose steps up. The order you enter doses does not matter, and a dose dated in the future adds nothing until its time comes.

Amount, not concentration

Reko shows an amount in the body, not a blood concentration. Concentration would need a volume of distribution, which varies with body size and is unknown for many compounds. An amount is the more honest quantity to show.

Where the half-lives come from

Elimination half-lives for licensed medicines are well published. Absorption half-lives are reported less consistently, so some are set so that the time to peak matches the published range. Where a figure is unsettled, Reko gives that compound no curve rather than a guess. For injected hormone esters, the half-life that matters is the apparent one from the oil depot, and published figures vary with the oil, the volume and the route.

[Insert half-life table per compound once each value is checked against its current label. verify against current label: elimination half-life, time to peak]

2. The reconstitution calculator

Most powdered peptides come in a vial that you mix with sterile water and draw up in a syringe. The sums are short.

1. Strength. Peptide mass divided by the volume of water added. 5 mg in 2 mL is 2.5 mg per mL.
2. Volume to draw. Your dose divided by the strength.
3. Syringe units. The volume times the units per mL of your syringe. A U-100 syringe has 100 units per mL; a U-40 syringe has 40.

Reko also shows how much medicine is in one syringe unit. That middle number lets you check the result by hand.

Why the syringe type has no default

You choose it every time. U-100 against U-40 is a factor of 2.5, so assuming the wrong one would mean drawing 2.5 times too much or too little.

Working backwards

Often you know the mark you would like to draw to, such as 10 units, and want to know how much water to add. Reko solves that direction too: peptide mass times the target units, divided by the dose times units per mL.

One simplification

Reko treats the final volume as equal to the water you add. The powder takes up a little space, but for a typical vial that effect is far smaller than the error of reading a syringe by eye.

The warnings

The calculator never refuses to answer and never hides the number. It adds a note when something looks risky:

• the draw is under 5 units, so reading error is large, or under 2 units, which is too small to read;
• the draw is over 50 units, which more often means a decimal slip than a real dose;
• the draw is more than the syringe holds;
• the draw falls between two printed marks, with the nearest mark shown;
• the solution is stronger than 10 mg per mL, so small measuring errors become large dose errors;
• the dose is more than what is left in the vial.

The suggested amount of water depends on the compound, the dose and the syringe's units per mL. It does not depend on the barrel size, so a 0.3 mL and a 1 mL U-100 syringe get the same advice.

See the vial tracker Free tools

3. How lab values and units are handled

A dose that is wrong by a factor of 1,000 is a bigger problem than a missing one, so Reko is cautious with numbers it did not type itself.

• One number reader. Every field in the app uses the same one. It trusts your phone's decimal separator and refuses a doubtful case, such as 1,500 or 1.500, with a message that names both readings.
• Fields are filled in plain form. Reko never writes a thousands separator into a field you are about to edit, so reopening a saved dose cannot change it.
• Lab scanning. Point the camera at a printed report. Reko reads the test names it knows and the value after each name, then shows every row for you to check. A row it is unsure of, such as a test with a similar name or an odd value, starts unticked with the reason beside it.
• Reference ranges. About sixty tests are stored with their units. They are plotted against your lab's own reference range, not a general one.
• Units. Pounds, stones or kilograms; fluid ounces or milliliters. Each compound keeps its own dose units.
• When a guess is unavoidable. A date like 01/05/2026 can mean two days. Reko shows how it read the file and asks you before saving.

4. What Reko cannot know

• How much medicine is in your blood. Only a lab test can tell you that.
• How your body absorbs and clears a compound. The curve uses published averages, and people differ.
• What is actually in a vial. Reko trusts the label and your entries.
• Whether a dose is right for you. Reko does not suggest doses.
• Anything you did not log. A missed entry is a gap in the curve.

Use the app to look back and to prepare questions. Bring those questions to whoever prescribed or supplied what you take.

Our standards

Each claim on this site is checked against how the app behaves. Sources are linked where we cite them. Where we have not yet checked a figure against its source, we say so on the page. See who reviews our work.

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