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Aromatase Inhibitors on steroid cycles

What are Aromatase Inhibitors (AIs) and their Medical Purpose?
Aromatase Inhibitors (AIs) are drugs that lower estrogen levels by stopping the aromatase (enzyme in fat tissue) from changing testosterone into estrogen.  Estrogen promotes the growth of breast epithelial cells, aka grows gynecomastia.  It does this by binding and activating the estrogen receptor.  The problem is that estrogen does not discriminate between normal healthy cells and cancerous cells.  From this we can see the importance of AIs in the medical world.  They are key in interrupting the development and progression of breast cancer.  Today, AIs are the first line of treatment for tumors in the breast.

aromatase inhibitors list

Fig 1. Aromatase Inhibitors List

Table of Contents

AI’s and Bodybuilding

Anabolic-Androgenic Steroids (AAS) are used by many bodybuilders to increase muscle growth and strength.  They do this while allowing them to burn fat at the same time.   AAS are structurally related to testosterone, and therefore most convert to estrogen.

As a result, males that have a high amount of estrogen in the body can develop gynecomastia.   Gynecomastia or “bitch tits” is male breast enlargement with lumps behind or under the nipples. This is where Aromatase Inhibitors (AIs) come in.  When a steroid user uses an aromatase inhibitor, they are able to keep their estrogen levels down and prevent gynecomastia.

AIs are also part of treatment plans with respect to gynecomastia.  The anti-gyno cycle 3.0 is an example of a treatment plan.  https://www.evolutionary.org/anti-gynecomastia-cure-version-3  Ultimately, if the gyno problem is bad enough, surgery will be required, so prevention is key.

Types of AI’s

We are currently on the third generation of aromatase inhibitors.  Each generation has been associated with a better ability to specifically bind to the aromatase enzyme, fewer negative side effects, and greater suppression of the aromatase activity. The three main AIs used today are:

  • Exemestane (Aromasin)Exemestane is a “suicide AI”, meaning that once it binds to the aromatase enzyme the bond is permanent.  There is no chance of the bound enzymes rebounding.  This is what makes it different from Anastrozole and Letrozole.  Side effects may include hot flashes, sweating, insomnia, headache, and joint pain.  A typical bodybuilder will dose Aromasin at 10mg EOD (every other day).
novartis letrozole

Fig 2. Novartis Letrozole

  • Anastrozole (Arimidex) – Anastrozole blocks the aromatase enzyme, which prevents the production of estrogen. When it binds to the aromatase enzyme, it forms a reversible bond.  Therefore, there is possibility of estrogen rebound with Anastrozole.  The main side effect associated with use is bone weakness.  A typical bodybuilder will dose Arimidex at .5mg EOD (every other day).
  • Letrozole (Femara)Letrozole works the same way as Anastrozole, but it's much more powerful.  The bond it forms with the aromatase enzyme is also reversible but is competitive as well.  This gives Letrozole the ability to crush estrogen levels quickly.  In fact, a typical bodybuilder will only use Letrozole if they develop gynecomastia or have to get their estrogen levels down very quickly.  Dosing varies and would start at 0.25mgs EOD and may increase as high as 1mg ED.

When Should I Use an AI on Cycle?

When a bodybuilder is running a steroid cycle with compounds that aromatize to estrogen, it is imperative to use an aromatase inhibitor from day one of the cycle.  The side effects associated with high estrogen are much easier to prevent than to deal with after symptoms arise.

aromatase inhibitor conversion

Fig 3. Testosterone to Estrogen

There is a common theory (broscience) that you can keep your AI “on hand” and use if needed.  This is a a very dangerous theory that leads to many side effect, including blood pressure and gyno.  You should always use aromatase inhibitors on cycle, especially during cycles of testosterone, deca durabolin, dbol etc.

This is why blood work is very important.  Truth be told, the only way to know where your estrogen levels are is through blood work.  It will tell you if your AI is legit, where your levels are at, and many other things about your body.  Remember that everyone responds differently to different compounds and this includes AIs.  You may need a much lower or higher dose than someone else.


Want to read about Aromatase Inhibitors on our forums? check out these threads:

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Matt Ashlar is a full time dad and an amazing bodybuilder. He has over 10 years of experience in bodybuilding, fitness, TRT and sarms. He's been at the forefront of supplement design and industry leaders. He has trained many bros in the community and helped guys iron out their cycles. He's a true PhD in our industry. He has the knowledge and the looks.

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