Half-life is most of the story
Injected testosterone is esterified so that it leaves the oil depot slowly. The hormone is the same; the ester sets the release rate. The FDA-approved label for Depo-Testosterone states it directly: "the half-life of testosterone cypionate when injected intramuscularly is approximately eight days."
Half-life is the time for the amount remaining to halve. It is the single number that decides how long a dose is still contributing when the next one lands — which is what produces accumulation, and what determines how far levels swing between injections.
| Ester | Elimination half-life in TRT AI's model | Note |
|---|---|---|
| Testosterone cypionate | 8 days (192 h) | Matches the "approximately eight days" in the FDA label. |
| Testosterone enanthate | 7 days (168 h) | Published estimates vary between studies and assays. |
| Testosterone propionate | 2 days (48 h) | Short ester — much larger swing at the same interval. |
| Sustanon 250 (blend) | 10 days (240 h) | A blend of esters modelled as one composite curve, which is a simplification. |
Those are the constants in TRT AI's compound directory, used for both the in-app chart and the half-life and steady-state calculator. They are model inputs, not measurements of you, and the literature genuinely disagrees at the edges.
Why levels keep climbing for weeks
If the next injection arrives while the previous one is still releasing, the two overlap and the total builds. This continues until what leaves per interval equals what arrives per interval — the state conventionally described as steady state, reached after roughly four to five elimination half-lives.
With an eight-day half-life that is on the order of a month or more. TRT AI's model puts the 95%-of-final-trough point at about 35 days for weekly cypionate and about 35 days when the same weekly total is split across two injections. The practical consequence is the same either way: a protocol changed this week cannot be fairly judged by a draw next week, which is why guideline monitoring windows are measured in months rather than days.
Frequency changes the swing, not the average
Here is the part that surprises people. Splitting the same weekly milligrams into more frequent, smaller injections does not raise the average level — the amount arriving per week is unchanged. What changes is the peak-to-trough ratio: the peak comes down, the trough comes up, and the line flattens.
Run through TRT AI's own engine, holding 100 mg of cypionate per week constant:
| Schedule | Weekly total | Steady-state peak-to-trough ratio |
|---|---|---|
| 100 mg once weekly | 100 mg | 1.40 : 1 |
| 50 mg every 3.5 days | 100 mg | 1.11 : 1 |
| ~14 mg daily | 100 mg | 1.01 : 1 |
Same milligrams, three different curves. This is arithmetic about a model, not a recommendation: how often you inject, and whether a flatter curve is desirable in your case, is a decision for the clinician who prescribes it. Frequency also trades against practicalities — more injections mean more sites, more supplies and more opportunities to miss one.
You can reproduce any of these curves, for any ester and interval, in the half-life and steady-state calculator.
What the model does not know
The curve is a first-order two-compartment (Bateman) model with dose superposition: an absorption ramp off the depot, then exponential elimination, summed across doses. It deliberately does not include bioavailability, volume of distribution, clearance, SHBG, body composition, injection site, or the individual variation that makes two men on identical protocols land in different places.
That is why the y-axis is relative and unlabelled rather than expressed in ng/dL. A calculator that promises you a serum concentration is overstating what pharmacokinetic constants alone can deliver. Use the shape — how long accumulation takes, how much the line swings — and get the actual concentration from a blood draw.
Where your blood draw sits in the interval
Because levels move across the interval, when you draw partly determines what the number says. Many clinics standardise on a trough draw immediately before the next injection, because it is easy to define and easy to repeat; the Endocrine Society guideline discusses measuring serum testosterone midway between injections for men on enanthate or cypionate. Both conventions exist, and your prescriber's protocol wins.
What matters for your own trend is that you keep doing the same thing and record it. A trough draw compared against a mid-interval draw is not a change in your protocol — it is a change in your measurement. Details in the lab timing guide.
Common questions
Does injecting more often raise my testosterone?
Not on its own. In the model, splitting the same weekly milligrams into more frequent doses leaves the average roughly unchanged and narrows the gap between peak and trough. A higher average requires more milligrams per week, which is a prescribing decision.
How long until a protocol change shows up in bloodwork?
For a long ester the model puts near-steady-state at roughly four to five elimination half-lives — weeks, not days. Drawing earlier measures a system still in transit, which is one reason guideline monitoring windows sit at 3–6 months.
Can this calculator tell me my ng/dL?
No, and no calculator honestly can. Absorption, bioavailability and clearance vary between individuals and are not inputs to a half-life model. The chart shows shape and relative accumulation; concentrations come from a lab.