What Is an Aromatase Inhibitor? AIs on TRT Explained
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Aromatase inhibitor (AI)

Also called AI, estrogen blocker, anastrozole, exemestane · Reviewed

Definition: An aromatase inhibitor (AI) is a drug that blocks aromatase, the enzyme that turns testosterone into estradiol (the main estrogen). Anastrozole (Arimidex) and exemestane are examples. Anastrozole is FDA approved for breast cancer in postmenopausal women; use on TRT to lower estradiol is off-label.

How it works

The Arimidex label dose is one 1 mg tablet once a day. It says aromatase converts androstenedione and testosterone into estrone and estradiol. In postmenopausal women, 1 mg of anastrozole daily lowered estradiol by about 70% within 24 hours and about 80% after 14 days. Those numbers come from women with breast cancer, not men on TRT.

The label also warns that bone mineral density may drop. The case for and against an AI on TRT is covered in Estradiol (E2) on TRT.

Worked example: a 50 hour half-life

How long a dose hangs around

  1. The Arimidex label gives a mean elimination half-life of 50 hours.
  2. A tablet logged Monday at 8 am is about half cleared by Wednesday at 10 am (50 hours later).
  3. About a quarter is left by Friday at noon (100 hours), and about an eighth by Sunday at 2 pm (150 hours).
  4. So the effect of one tablet lasts days, and changes to how often it is taken show up slowly in blood work.

Why it matters when you track doses

Log the amount and the day of each AI dose, and note the date of any estradiol test, so you and your prescriber can line up changes with lab results.

Read more on Dose Track: Anastrozole · Exemestane · Half-life calculator

General education drawn from labels and published studies, not personal medical advice. Your prescriber, pharmacist and the label that came with your product are the final word.
Half-lifeHalf-life is the time it takes for the amount of a drug in your body to fall by half. After one half-life,… Testosterone esterA testosterone ester is testosterone with a fatty acid chain attached, such as cypionate or enanthate. The… Steady stateSteady state is the point, after repeated doses on a fixed schedule, where the amount cleared between… HematocritHematocrit is the percentage of your blood volume made up of red blood cells. A result of 45 means about…

Frequently asked questions

Is anastrozole approved for men?+
No. The Arimidex label approves it only for breast cancer in postmenopausal women. Use on TRT is off-label and a decision for you and your prescriber.
Can an aromatase inhibitor lower estradiol too much?+
Yes. The Arimidex label reports large drops in estradiol and warns that bone mineral density may decrease. Those data come from women with breast cancer.

Sources

  1. Arimidex (anastrozole) US prescribing information, DailyMed

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