Testosterone Cypionate half-life and steady state
6 days modelled half-life, peaking 2 days after a dose. What that means for how long it lasts, how much it builds up, and when bloodwork is worth drawing.
- Also known as
- Test C, TC
- Class
- Androgens / TRT
- Modelled half-life
- 6 days
- Time to peak
- 2 days
- Formulation
- Oil-based (intramuscular or subcutaneous)
- Cleared per dosing interval
- 33% gone, 67% still present after 3.5 days
- Time to steady state
- ~30 days (about five half-lives)
- Accumulation at that cadence
- 3.01× a single dose
- Peak-to-trough at steady state
- 1.21×
- Regulatory status
- Off-label dosing above TRT levels. TRT itself requires prescription.
- Monitoring panel
- Total T, Free T, E2 (sensitive assay), SHBG, Hematocrit, CBC, PSA, Lipids, CMP. Every 6-8 weeks adjusting; every 3-6 months stable.
- Storage
- Room temperature, 20–25 °C, out of direct light.
- Once opened
- Same once opened. Oil-based vials do not get a fridge.
- What ruins it
- Do not refrigerate. Cold thickens the carrier and can bring the ester out of solution. If you see crystals or cloudiness, warm the vial in your hand or a cup of warm water and look again — if it does not clear, do not use it.
- Handling caveat
- General handling practice for this formulation. Your supplier’s own insert or COA overrides anything here.
Testosterone cypionate's half-life is a property of the ester, not of testosterone. Testosterone itself has a half-life measured in tens of minutes. Attaching an eight-carbon cypionate chain makes the molecule oil-soluble, so an intramuscular or subcutaneous depot releases it slowly, and the enzymes that cleave the ester become the rate-limiting step. The roughly six-day figure is how fast that depot empties, which is why the model's time-to-peak is two full days rather than the minutes you would see from an unesterified injection.
That release-limited behaviour is what makes the cadence question real. At twice weekly the modelled peak-to-trough swing is close to flat; at once weekly it roughly doubles, while the accumulation falls. Neither is wrong — they are different curves for the same weekly milligrams, and which one suits a person is a clinical judgement rather than an arithmetic one.
Two things follow for bloodwork, and they cause more confused panels than anything else in TRT. First, a level takes about a month to settle after any change, so labs drawn a fortnight into a new dose are measuring a number still climbing. Second, when you draw matters: a peak-time draw and a trough draw on the same protocol differ by the ratio on this page, so a result without a stated draw time relative to the last injection is hard to compare against anything. The convention that makes results comparable is a trough draw, immediately before the next injection.
One dose
pkCurve function — a
one-compartment absorption-and-elimination model with the absorption rate fitted so the
peak lands at the published time to peak.Dosed twice weekly
Charted at twice weekly because that is one of the schedules in the app's own dosing rows, and the most common in practice. It is what the numbers below are indexed to, not a recommendation about frequency — that is a prescribing decision.
The arithmetic behind those numbers
fraction left after one interval = 2^(−0.58) = 0.667
accumulation ratio = 1 ÷ (1 − 0.667) = 3.01
time to steady state ≈ 5 × half-life = 30 days
peak-to-trough = simulated, 3.327 ÷ 2.7496 = 1.21
The accumulation ratio is a closed form; the peak-to-trough figure is not. The generator adds up enough repeated curves for the total to stop changing, then reads the highest and lowest points of the last interval — which is what steady state means in practice. The vertical axis is relative, not a concentration: the shape and the ratios carry across people, the absolute levels do not.
Interaction rules that name Testosterone Cypionate
From the app's own interaction data. Not exhaustive, and not a safety clearance — a combination that is not listed is one nobody has documented here, which is not the same as one that is fine. The full combination checker has the rest.
DHT Suppression Trade-offs With TRT
Dutasteride suppresses ~90% of DHT conversion from testosterone. This prevents hair loss but reduces libido, erection quality, mood, and muscle fullness in some men. The trade-off is significant — weigh hair preservation vs androgenic benefits of DHT.
What to watch: Total T, Free T, DHT if testable, libido and sexual function self-assessment monthly.
DHT Suppression Trade-offs With TRT
Finasteride suppresses ~70% of DHT. Risk of Post-Finasteride Syndrome (persistent sexual dysfunction) exists in a subset of users. If symptoms develop, discontinue immediately.
What to watch: Total T, Free T, libido and sexual function. Discontinue if persistent sexual or mood side effects occur.
GLP-1 + Testosterone — Essential Muscle Preservation
GLP-1 agents cause significant muscle loss without adequate testosterone support. Running TRT-dose testosterone alongside semaglutide is strongly recommended to preserve lean mass during weight loss. Resistance training is also essential.
What to watch: Body composition (DEXA preferred), Total T, HbA1c, fasting glucose, lipids.
GLP-1 + Testosterone — Essential Muscle Preservation
Same as semaglutide — tirzepatide's superior fat loss effect also means superior muscle loss risk without testosterone support. TRT-dose testosterone is strongly recommended alongside any GLP-1 agent.
What to watch: Body composition (DEXA preferred), Total T, HbA1c, fasting glucose, lipids.
T3 + Testosterone — Metabolic Synergy
T3 and testosterone work synergistically for body composition. T3 increases metabolic rate and fat oxidation while testosterone preserves and builds lean mass. A common optimization combination. Monitor carefully as T3 can increase protein catabolism at high doses.
What to watch: Free T3, Free T4, TSH, Total T, heart rate (resting), body composition.
DHEA + Testosterone — Verify No Excess Estrogen
DHEA can convert to both testosterone and estrogen. Adding DHEA to a TRT protocol may raise estrogen further. Monitor E2 closely and do not add DHEA without baseline DHEA-S testing.
What to watch: DHEA-S, Total T, Free T, E2 (sensitive assay), PSA. Recheck 6 weeks after adding DHEA.
Metformin + TRT — Insulin Sensitivity Benefits
Metformin improves insulin sensitivity and activates AMPK — complementary to testosterone's anabolic effects. Some research suggests metformin may slightly reduce testosterone levels; ensure adequate TRT dose and monitor labs.
What to watch: Total T, Free T, HbA1c, fasting glucose, Vitamin B12 (metformin depletes B12 — supplement). Recheck labs at 8 weeks.
Telmisartan + TRT — Recommended Cardiovascular Protection
TRT can raise blood pressure and hematocrit. Telmisartan addresses both concerns via angiotensin blockade and its PPAR-gamma activity improves insulin sensitivity. A thoughtful pairing for TRT patients with BP concerns.
What to watch: Blood pressure, hematocrit, potassium, eGFR. Target BP under 130/80.
MT-2 + TRT — Monitor Skin and E2
MT-2 can darken existing moles and nevi. On TRT, where estrogen can also affect skin, monitoring skin changes is important. E2 optimization is also important as both estrogen and melanocortin pathways affect libido — don't over-suppress E2 if also using MT-2 for sexual function.
What to watch: Dermatology skin check baseline and every 6 months. E2 (sensitive assay) — maintain 20-35 pg/mL for optimal libido synergy.
Rapamycin + TRT — Monitor for mTOR Effects on Muscle
mTOR is required for muscle protein synthesis. Rapamycin's mTOR inhibition could theoretically blunt anabolic effects of testosterone. At weekly longevity doses (5-10mg), this effect is minimal due to intermittent dosing. Ensure adequate protein intake and resistance training.
What to watch: Body composition, lean mass tracking, strength metrics. Dose rapamycin weekly (not daily) to minimize muscle synthesis blunting.
The dosing rows this cadence came from
Off-label or community practice Reproduced from the app's entry so you can see what the chart above is indexed to. Which row applies to a particular person, if any, is a clinical decision this page does not make.
| Label | Amount | Route and frequency |
|---|---|---|
| TRT / Starting | 100-150mg/week | Once or twice weekly IM/SubQ |
| Optimization | 150-200mg/week | Twice weekly preferred |
When to draw bloodwork
The panel the app records for Testosterone Cypionate is: Total T, Free T, E2 (sensitive assay), SHBG, Hematocrit, CBC, PSA, Lipids, CMP. Every 6-8 weeks adjusting; every 3-6 months stable.. The pharmacokinetic point is the timing rather than the list: after any change, a level takes about 30 days to settle, so a panel drawn sooner measures something still moving. The swing at this cadence is small (1.21×), so draw timing within the interval matters less here than it does for a short-acting compound. Whether a result means anything is a question for the clinician who ordered it, read against the reference range your own lab printed. The lab-marker pages cover what the individual analytes measure.
Related: the interactive half-life calculator to try other cadences.
The app draws this curve from the doses you actually logged — your dates, your cadence — and projects seven days forward from the last one.
Save this dose to your log