Anastrozole half-life and steady state
46 h modelled half-life, peaking 2 h 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
- Arimidex
- Class
- Ancillaries — AIs and SERMs
- Modelled half-life
- 46 h
- Time to peak
- 2 h
- Formulation
- Oral
- Cleared per dosing interval
- 66% gone, 34% still present after 3 days
- Time to steady state
- ~9.6 days (about five half-lives)
- Accumulation at that cadence
- 1.51× a single dose
- Peak-to-trough at steady state
- 2.86×
- Regulatory status
- FDA approved for breast cancer. Off-label for estrogen management in TRT.
- Monitoring panel
- E2 sensitive assay, Total T, Free T every 6-8 weeks when adjusting.
- Storage
- Room temperature and dry, in the original container with whatever desiccant came in it, out of light.
- Once opened
- Unchanged once opened, as long as it stays dry.
- What ruins it
- Humidity is the real enemy — a bathroom cabinet is the worst place in the house for these.
- Handling caveat
- General handling practice for this formulation. Your supplier’s own insert or COA overrides anything here.
Anastrozole's roughly two-day half-life is the reason it is dosed every third day for oestrogen management rather than daily as it is in oncology. At that interval each dose lands on a substantial remainder of the last, so the level accumulates and the aromatase inhibition is continuous rather than intermittent — which is the intent, since partial suppression of a continuously running enzyme is the goal.
The pharmacokinetic subtlety is that the drug's half-life and its effect's half-life are not the same. Anastrozole is a competitive, reversible inhibitor of aromatase, so inhibition tracks the drug level fairly closely — but the oestradiol measured in blood is the integral of production and clearance over hours to days, and it lags. A dose change shows up in an E2 result over the following week, not the following morning.
That lag is why the most consequential risk on this compound is over-correction, and why the app's entry carries a warning about it. Oestradiol is not a waste product in men: it is required for bone density, joint comfort, libido and cardiovascular protection, and crashing it produces a recognisable and unpleasant syndrome. Because anastrozole accumulates over several doses, a dose that seemed too weak in week one can be too strong by week three — the level is still rising while the judgement is being made. The monitoring the app specifies, a sensitive-assay E2 alongside total and free testosterone every six to eight weeks when adjusting, is timed to that. Anastrozole is approved for breast cancer; use for oestrogen management on testosterone therapy is off-label, and it is a prescription decision.
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 every three days
Charted at every three days because that is the every-third-day schedule in the app's own dosing rows. 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^(−1.57) = 0.338
accumulation ratio = 1 ÷ (1 − 0.338) = 1.51
time to steady state ≈ 5 × half-life = 9.6 days
peak-to-trough = simulated, 1.514 ÷ 0.5291 = 2.86
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 Anastrozole
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.
Do Not Double-Stack Aromatase Inhibitors
Using two AIs simultaneously causes severe estrogen crash leading to joint pain, depression, osteoporosis risk, and cardiovascular damage. Use one AI only, guided by bloodwork.
What to watch: E2 sensitive assay, Total T, bone density if long-term.
Comprehensive Hormone Management Stack
Using both an AI and a dopamine agonist together is appropriate when running progestogenic compounds like nandrolone that raise both estrogen and prolactin. Ensure each is dosed based on its respective lab value — E2 guides AI dose, prolactin guides Cabergoline dose.
What to watch: E2 (sensitive), Prolactin, Total T every 6-8 weeks.
DHEA May Partially Offset AI Effect
DHEA aromatizes to estrogen, potentially undermining the estrogen-lowering effect of anastrozole. If using both, E2 monitoring is especially important to confirm AI is achieving target levels.
What to watch: E2 sensitive assay every 6-8 weeks. Adjust anastrozole dose based on labs only.
The dosing rows this cadence came from
Established clinical use 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 |
|---|---|---|
| Conservative | 0.25mg E3D | Every 3 days oral |
| Standard | 0.5mg E3D | Every 3 days |
| Higher dose | 1mg E3D | Bloodwork guidance only |
When to draw bloodwork
The panel the app records for Anastrozole is: E2 sensitive assay, Total T, Free T every 6-8 weeks when adjusting.. The pharmacokinetic point is the timing rather than the list: after any change, a level takes about 9.6 days to settle, so a panel drawn sooner measures something still moving. And because the peak-to-trough swing here is 2.86×, when in the interval you draw changes the result materially — a result without a stated draw time relative to the last dose is hard to compare with anything. 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