Half-life, explained

Half-life is the single number that explains why a daily peptide and a once-a-month testosterone shot behave so differently in your body — and why the wrong injection schedule can leave you riding peaks and troughs instead of a steady line.

Ask why a testosterone cypionate protocol calls for a shot every one to two weeks, while BPC-157 protocols call for injections once or even twice a day, and the answer always traces back to the same variable: half-life. It sounds like a pharmacology-class term, but the concept is simple once you see it laid out, and it explains almost everything about why injection schedules look the way they do — for peptides and for testosterone esters alike.

What half-life actually means

Half-life (t½) is the time it takes for the amount of a substance in your body to fall by 50%.¹ That's it — no more complicated than that. If a compound has a 4-day half-life and you start with 100 units in your system, you'll have about 50 units left after 4 days, 25 units after 8 days, roughly 12.5 after 12 days, and so on. Each half-life cuts what's left in half again, regardless of how much you started with. A very short half-life (minutes to hours) means the compound clears fast and needs frequent redosing to stay active; a very long one (days to weeks) means a single dose keeps contributing to your blood level long after you've forgotten about it.

Exponential decay: why the drop isn't a straight line

Most drugs, testosterone esters, and peptides follow what's called first-order elimination — the rate of clearance is proportional to how much is currently in your system, not a fixed amount per day.¹ That produces a curve, not a ramp: a steep drop early on that gradually flattens out, because there's simply less left to eliminate as time goes on. The practical version of the rule most clinicians use: roughly 90% of a dose is gone after about 3.3 half-lives, and 94–97% is gone after 4–5 half-lives.¹ That last sliver — the long, flat tail of the curve — is also why a compound is never fully "out of your system" the day after your last dose. It's just down to a fraction that keeps halving.

Why half-life determines how doses stack

Here's where half-life stops being an abstract number and starts shaping your actual protocol. Every time you inject before the previous dose has fully cleared, the new dose lands on top of whatever's left — the two amounts add together. Inject again before that combined amount clears, and it stacks again. This is accumulation, and whether it happens (and how much) depends entirely on the relationship between your injection interval and the compound's half-life. Dose every half-life or faster, and each new shot adds meaningfully to a rising baseline. Dose only once every 4–5 half-lives, and the previous dose is essentially gone before the next one arrives — little to no stacking.

Steady state: what it is and how long it takes

"Steady state" is the point in a repeated-dosing schedule where the amount you're clearing between doses roughly equals the amount you're adding — your peaks and troughs still oscillate, but the average level stops climbing.² The rule of thumb pharmacology references use is consistent: steady state arrives after about 4–5 half-lives of consistent dosing, no matter how large or small the individual dose is.² That's why a testosterone propionate protocol (half-life under a day) can settle into a stable rhythm within about a week, while injectable testosterone undecanoate — half-life measured in weeks — can take two to three months of loading doses before levels stop climbing and plateau.

Interval vs half-life: how it shapes your peaks and troughs

Two protocols can deliver the exact same weekly total dose and still feel completely different, because how spread out the doses are relative to the half-life changes the shape of the curve. Inject a short-half-life compound infrequently, and you get a sharp peak right after the shot followed by a steep decline into a real trough before the next one — the pattern behind the "crash" some people describe a day or two before their next testosterone injection. Shrink the interval relative to the half-life (smaller, more frequent doses — sometimes called microdosing), and the peaks and troughs compress toward a flatter line, because less of each dose has cleared before the next one lands. We cover that trade-off in detail in our TRT microdosing guide, and the injection-route comparison in subcutaneous vs. intramuscular testosterone gets into how absorption speed adds a second layer on top of half-life itself — subcutaneous depots tend to release more gradually than intramuscular ones, which can flatten the curve further even at an identical dose and ester.

Accumulation factor: putting a number on it

Pharmacologists express how much a repeated dose builds up using an accumulation ratio (AR) — essentially, the steady-state average level divided by what a single dose alone would produce.² The key threshold to remember: meaningful accumulation shows up once your dosing interval is shorter than about four half-lives.² Space doses out beyond that, and each one behaves almost like a standalone dose with little carryover. That's the entire logic behind why a long-half-life compound like injectable testosterone undecanoate is dosed every 10 weeks instead of weekly — dosing it weekly, at a fraction of the depot dose, would produce runaway accumulation because 10 weeks is nowhere near four undecanoate half-lives.

Testosterone ester and peptide half-life cheat sheet

The table below pulls half-life figures directly from published pharmacokinetic studies for the testosterone esters most commonly used in TRT, plus a few reference peptides for comparison. Ranges reflect dose and study-to-study variation — see the numbered sources for exact study conditions.

What the table means for your injection schedule

Line the table up against the accumulation rule and the logic behind common protocols becomes obvious. Propionate's half-life is so short that skipping even a couple of days lets levels fall meaningfully — hence the every-other-day dosing.⁸ Cypionate and enanthate sit in the middle, which is exactly why the "weekly vs. every-two-weeks" debate exists: both intervals are within a few half-lives of each other for these esters, so the choice comes down to how much peak-to-trough swing you're willing to tolerate rather than any hard cutoff.³ Undecanoate's multi-week half-life is why its label calls for loading doses at week 0 and week 4 before settling into a 10-week maintenance interval — anything faster would stack the depot up far past steady state.⁵ On the peptide side, a compound like BPC-157 with a half-life measured in hours needs multiple small doses spread through the day to keep any meaningful level in circulation at all, while CJC-1295's roughly 30-minute half-life (without a DAC extension) is why most protocols time doses tightly around specific windows rather than trying to hold a steady baseline.

Peptide half-lives: why some need daily dosing and others don't

The same math explains the wide spread you'll see across our peptide database. Unmodified peptides are usually cleared by the kidneys and by circulating enzymes within minutes to a few hours — a review of peptide and protein conjugates cites bombesin analogs at roughly 0.5–1.5 hours and somatostatin analogs at about 90 minutes.⁷ That's why growth-hormone secretagogues and many research peptides are dosed once or several times a day: there's nothing structurally protecting them from rapid clearance. Longer-acting peptide drugs get there through deliberate half-life-extension chemistry — the same review notes that unconjugated interferon alfa clears in 2–3 hours and would need daily injections, but conjugating it to albumin stretches its half-life to roughly 6 days, enabling dosing every 2–4 weeks instead.⁷ GLP-1 drugs like semaglutide use a similar fatty-acid/albumin-binding strategy to get to a roughly weekly dosing interval. The takeaway: a peptide's dosing frequency isn't arbitrary — it's a direct readout of how long the specific molecule survives in circulation.

Using a half-life calculator to plan your protocol

All of this — exponential decay, steady state, accumulation — is easier to reason about as a curve than as a set of rules to memorize. That's what our half-life calculator does: enter a compound (or a custom half-life), your dose, and your interval, and it projects the accumulation curve so you can see where your peaks, troughs, and steady-state average will actually land before you commit to a schedule. Inside the Peptide Me app, the same engine runs against your logged dose history to forecast your personal blood-level curve going forward — so instead of estimating from a table, you're looking at a projection built from the doses and timing you actually recorded.

The bottom line

Half-life is the hidden variable behind almost every injection-schedule question — why some protocols call for daily shots and others for a single injection every ten weeks, why switching intervals changes how "steady" you feel, and how long you should wait before your first post-start bloodwork actually reflects a stable state rather than a still-climbing one. None of the figures above replace an individualized plan: talk to a licensed prescriber about your specific protocol, especially before changing an established testosterone or peptide dosing schedule, and treat this article as educational background rather than medical advice.

FAQ

References

Primary sources — PubMed / NEJM / The Lancet.

1. Hallare J, Gerriets V. Elimination Half-Life of Drugs. StatPearls. 2025. https://www.ncbi.nlm.nih.gov/books/NBK554498/
2. Wadhwa RR, Cascella M. Steady State Concentration. StatPearls. 2023. https://www.ncbi.nlm.nih.gov/books/NBK553132/
3. Bi Y, Perry PJ, Ellerby M, Murry DJ. Population Pharmacokinetic/Pharmacodynamic Modeling of Depot Testosterone Cypionate in Healthy Male Subjects. CPT Pharmacometrics Syst Pharmacol. 2018;7(4):259-268. https://pmc.ncbi.nlm.nih.gov/articles/PMC5915615/
4. Kaminetsky J, Jaffe JS, Swerdloff RS. Pharmacokinetic Profile of Subcutaneous Testosterone Enanthate Delivered via a Novel, Prefilled Single-Use Autoinjector: A Phase II Study. Sex Med. 2015;3(4):269-279. https://pmc.ncbi.nlm.nih.gov/articles/PMC4721027/
5. Zhang GY, Gu YQ, Wang XH, Cui YG, Bremner WJ. A Pharmacokinetic Study of Injectable Testosterone Undecanoate in Hypogonadal Men. J Androl. 1998;19(6):761-768. https://pubmed.ncbi.nlm.nih.gov/9876028/
6. Pastuszak AW, Gittelman M, Tursi JP, et al. Pharmacokinetics of Testosterone Therapies in Relation to Diurnal Variation of Serum Testosterone Levels as Men Age. Andrology. 2022;10(2):209-222. https://pmc.ncbi.nlm.nih.gov/articles/PMC9293229/
7. Bumbaca B, Li Z, Shah DK. Pharmacokinetics of Protein and Peptide Conjugates. Drug Metab Pharmacokinet. 2019;34(1):42-54. https://pmc.ncbi.nlm.nih.gov/articles/PMC6378135/
8. Shinohara Y, Fujioka M, Baba S. Pharmacokinetic Studies of Testosterone Propionate. Biomed Environ Mass Spectrom. 1988;16(1-12):241-244. https://pubmed.ncbi.nlm.nih.gov/3242676/