By Austin Witherow and Mason Calvert · Last updated · Sources
What a half-life is
A half-life is the time it takes for half of what is in the body to clear. It does not mean the drug is gone after two half-lives. Each half-life removes half of what is left, so the amount shrinks by the same share every time, not by the same number of milligrams.
| Half-lives passed | Share left of one dose |
|---|---|
| 1 | 50% |
| 2 | 25% |
| 3 | 12.5% |
| 4 | 6.3% |
| 5 | 3.1% |
After five half-lives about 3.1% of a single dose is left. For Test E, with a half-life of 4.5 days, five half-lives is 22.5 days.
Why levels climb, then level off
When doses repeat, each new dose lands on top of what is left of the earlier ones. At first more arrives each week than clears, so the curve climbs. As the total grows, the amount clearing each week grows with it, until it matches what each dose adds. From then on the curve repeats the same shape. That point is called steady state.
Read off the plotter's own curve, 400 mg of Test E once a week levels off around day 28. That is 6.2 half-lives. The common rule of thumb is about five; Pedplot's test is stricter, counting a schedule as levelled off only at the first dose whose low point is within 3% of where it settles.
Peaks, troughs and swing
Once a schedule has levelled off, each dose interval has a high point (the peak) and a low point just before the next dose (the trough). Swing is the gap between them as a share of the peak: (peak − trough) ÷ peak. A swing of 0% would be a flat line.
The swing depends on how long the gap between doses is compared with the half-life. A gap much shorter than the half-life leaves most of each dose in place when the next one arrives, so the line stays flatter.
The same weekly amount, split three ways
This is 400 mg of Test E a week, the plotter's example input, given once a week, twice a week, or every 3 days for 12 weeks. The table reads the last dose interval of each run.
| Schedule | Each dose | Peak | Trough | Swing | Plotter |
|---|---|---|---|---|---|
| once a week | 400 mg | 436 mg | 202 mg | 54% | Open |
| twice a week | 200 mg | 346 mg | 274 mg | 21% | Open |
| every 3 days | 171.4 mg | 333 mg | 286 mg | 14% | Open |
- 400 mg once a week
- 200 mg twice a week
- 171.4 mg every 3 days
Once a week, the swing is 54%. Twice a week it drops to 21%, and every 3 days to 14%. The middle of the range barely moves: the weekly amount is the same, so the average is the same. What changes is how far the line rises above and falls below it.
Every other day is not in the table. The model draws each dose as a straight rise over 2 days, so with a 2-day gap every dose is still rising when the next one lands, and the line comes out flatter than any real curve.
Long and short esters
The ester attached to a hormone sets how fast it leaves the injection site, which is what the half-life here describes. Compare one 100 mg dose of Test E (half-life 4.5 days) with one 100 mg dose of Test P (half-life 19 hours, a community estimate with no human study behind it).
| One 100 mg dose | Half-life | Peak | Peak on day | Down to 10% by day |
|---|---|---|---|---|
| Test E | 4.5 days | 72 mg | 2 | 17 |
| Test P | 19 hours | 84 mg | 1 | 3.75 |
- 100 mg Test E
- 100 mg Test P
Test P peaks higher, at 84 mg against 72 mg, because its ester is lighter and more of each milligram is testosterone (see the ester weight guide). It is also gone much sooner: down to 10% of its peak by day 3.75, against day 17 for Test E. That is why short esters are plotted with short gaps between doses.
What this does not show
These curves are the modeled amount in the body, not a blood level, and real absorption varies with the injection site, the oil and the person. See how the model works for the formula and its limits.
Sources
The model and its limits are on the methodology page.