How estimates work

Each point on the line is an estimate of the amount of medicine, in milligrams, still in your body at that moment. It is worked out from two things: the doses you have logged, with their times, and published figures for how quickly each medicine is absorbed and cleared in adults on average.

It is an estimate from population averages, not a measurement of your blood. It does not show how strongly the medicine is acting, how hungry you will feel or what your weight will do. People differ, and the chart cannot know how you differ. It is there to help you see the shape of a schedule, for example why a steady weekly dose settles into a plateau, or what a late dose does to the curve.

The app uses a one compartment model with first order absorption and elimination, and it adds up the contribution of every dose you have logged.

• One compartment means the body is treated as a single well mixed space. Medicine enters it from the injection site and leaves it as the body clears it.
• First order absorption means the medicine moves from the injection site into the blood at a rate proportional to how much is still waiting at the site, so the inflow is fastest just after the jab and tails off.
• First order elimination means the body clears a fixed fraction of what is present in each unit of time. The half-life is the time it takes for that amount to fall by half. With a half-life of about 7 days, about half of what was in the body a week ago is still there today, before the next dose adds to it.
• Bioavailability is the share of an injected dose that reaches the bloodstream. The model multiplies each dose by it.
• Summing every dose means each logged dose has its own curve, and the chart shows the total. That is why levels climb over the first few weeks of a steady schedule and then flatten: each new dose adds about as much as the older doses lose.

The absorption rate is not published directly for these medicines, so the app solves for it from the published peak time: the time after a dose at which the amount in the body is highest.

These are the values the app uses for the weekly injections. All of them are checked against the UK Summary of Product Characteristics (SmPC) before launch.

The symbols: t_1/2 is the half-life, k_e the elimination rate constant, k_a the absorption rate constant, t_max the time of the peak after a single dose, D_i the size of dose i in mg, t_i the time it was taken, F the bioavailability, and A(t) the estimated amount in the body at time t.

For semaglutide, with a half-life of about 165 hours, that gives k_e of about 0.0042 per hour.

The published t_max and the k_e from the first equation are known, so the app solves this equation numerically for k_a.

Each dose contributes nothing before it is taken, rises as it is absorbed, and then decays. The chart adds the terms for every logged dose and plots the total against time.

Retatrutide is investigational and holds no UK licence. The app lists it with an acknowledgement and accepts only doses you enter yourself. Because its bioavailability has not been published, the amount in milligrams cannot be estimated, so the chart shows each point as a percentage of your own highest estimated level. The shape of the curve is the same; only the scale differs.

Each parameter is stored with its source and a version number, and the sources are listed beside this page. A correction ships without an app update, and the chart notes which version produced it. Before launch, every value is re-checked against the UK SmPC for that medicine.

If a dose is late, the app shows how many days have passed since it was due, the wording from the patient leaflet for your region, and a link to that leaflet. It does not work out whether you should take the dose or skip it. Leaflets differ between countries (the UK and US Wegovy leaflets give different rules), so the wording is stored per region and the leaflet, not the app, is the authority.

• It is never written to Apple Health or Health Connect. Only the weight you enter yourself is shared, and only with your permission.
• It never recommends a dose or a dose change, and never ranks one dose against another.
• Dose Insights that sit alongside it describe each dose period (weight trend, symptom days, on-time rate) and stop there.
• It never replaces a conversation with your prescriber.

Questions about the method are welcome through the contact form.