How long do I have to wait to re-test e2 after changing arimidex doseage?

I really need a straight, scientific answer to this. I've read various responses to this question on this and other forums. They range widely from 10 days to 3 weeks to 6 weeks. All of them seem to just be wild guesses with no actual data to back them up.

I'm sure that there's some way to definitively calculate the time for arimidex to reach steady state based on it's halflife and frequency of dosage. I'm assuming that the answer would change based on whether it's taken 2x per week, 3x per week, or every other day.

Can anyone here help me figure out the real answer to this question? I'm currently waiting 3 weeks between dosage changes and re-tests, and it's killing me. I change my arimidex doseage, wait 3 weeks, get labs drawn, wait a week for the results. That's 1 month out of my life feeling like crap and unable to get it up for every time I try a doseage that doesn't work. The months are starting to add up. If it really takes that long for arimidex to build up in my system and stabilize e2 at a new level then I'll keep at it. If it's possible to change the dose and then re-test sooner that would save me a LOT of time and frustration.

So, someone please help me. How long should I be waiting after increasing or decreasing my arimidex dosage before I re-test e2 levels?

I found this in the prescribing info for Arimidex. It basically says that at once daily dosing steady state is acheived in 7 days. I'm assuming that taking it 2 or 3 times per week instead of once daily would change this and it would take longer to reach steady state. Can anyone confirm?

Anastrozole is eliminated slowly with a plasma elimination half-life of approximately 50 hours in postmenopausal women. The pharmacokinetics of anastrozole are linear over the dose range of 1 to 20 mg and do not change with repeated dosing. Consistent with the 50 hour plasma elimination half-life, plasma concentrations of anastrozole approach steady-state concentrations after 7 days of once daily dosing and are approximately 3- to 4-fold higher than the concentrations observed after a single dose of anastrozole.

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Sorry to keep replying to myself. I may have answered my own question. According to Dr. Google, steady state of a drug is achieved after 4-5 half-lives, and frequency of administration doesn't affect the time to reach steady state.

So, assuming I'm not missing anything I can figure out when my e2 will be at it's new stable level after changing doses by calculating the time for arimidex to reach it's new steady state, plus the time for estradiol left over from the old arimidex level to be eliminated:


Arimidex half-life of 46.8 hours x 5 half-lives = 234 hours to reach steady state

Estradiol half-life of 15 hours x 5 half-lives = 75 hours to eliminate leftover estradiol.

234 hours + 75 hours = 309 hours, or around 13 days.

Does all of that seem correct? Based on the above it seems to me that I'd be pretty safe re-testing e2 levels 2 weeks after changing dosages. Thoughts?

I speculate 2 weeks may be good enough, but I really have not seen receptor occupancy data on anastrozole to say for sure.

I would do two weeks but that'd be a minimum and I'd probably only do it if having made a small and singular change.

Vince Carter said:

I would do two weeks but that'd be a minimum and I'd probably only do it if having made a small and singular change.

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On what do you base this recommendation?

I think a major player in why you ask this question and are having problems finding the answer you seek is to remember that Arimidex as we use it, it's off-label; not approved. Thus you can only find personal and/or very informed experience. The best info I've ever seen on Arimidex (and Anastrozole) is it's half-life, 50hrs, and then knowing by experience it takes weeks for hormones to stabilize.

HRT is a journey of time, fine tune, test, tune, re-test. No one hits on a working protocol for themselves in days or even months. Most anyone that is going to say that they're in a steady effective state, and protocol, has a year or more under their belt.