TL:DR SERMs and AIs are not to be confused, though are sometimes mentioned interchangeably. Both can have roles in a cycle.

What is a SERM?

Firstly, what is a SERM? SERM stands for selective estrogen receptor modulator and primarily has an impact on estradiol levels.

Compounds that modulate estradiol levels in these clinical conditions are referred to as selective estrogen receptor modulators (SERMs)... In a certain subset of infertile men, particularly those with hypogonadism, or those who have a low serum testosterone to estradiol ratio, there is some evidence suggesting that SERMs... can reverse the low serum testosterone levels or the testosterone to estradiol imbalance and occasionally improve any associated infertile or subfertile state.

A SERMs role is to increase luteinizing hormone (LH) and follicle-stimulating hormone (FSH) that will increase testosterone:

(SERMs) work as estrogen antagonists at the level of the pituitary gland and thus stimulate the release of luteinizing hormone and follicle-stimulating hormone, which in turn drive both the steroidogenic and spermatogenic functions of the testes.

Or another way, SERMs bind to estrogen receptors, and can increase testosterone.

SERMs also increase SHBG through having an estrogen-like effect in the liver, which is where SHBG is created. Some SERMs are better than others though, with tamoxifen having a significantly greater impact than raloxifene. Effects are notdose dependent with no statistical difference between 10mg of Tamoxifen and 20mg - SHBG levels at 40.9nm/L and 38.9nm/L respectively.

What is an AI?

Developed for breast cancer treatment. As the name suggests, an Aromatase Inhibitor inhibits aromatase. Aromatase is an enzyme that naturally exists in everyone, and is what is responsible for androgens turning into estrogens. It’s found in body fat, blood, skin, bone, and gonads. Without an AI when exogenous testosterone is used, aromatase turns the androgen into estradiol (e2) efficiently.

While not spoken about regularly in these subs, AIs have been used to support growth (height) and treat gyno. For grown adults, the latter is not recommended - there are other known protocols. Please note that neither are perfect solutions, have a high amount of risk and a high failure rate.

What’s the difference between AI/SERM

SERMs prevent e2 from binding with receptors, and can help LH & FSH levels. An AI prevents test from becoming e2. In the context of PEDs, SERMs are used in PCT, treatment of gyno, or as an attempt to increase test while on a SARM only cycle (mixed data on that to date). An AI on the other hand is useful only when exogenous test is used, and is best practice to include even at TRT doses.

> But comic, an AI has sides. Why risk it on relatively low doses of test?

It’s about risk minimization. AIs are generally very well tolerated. Gyno is not (well tolerated that is). It can be devastating to self-esteem. Once you have developed breast tissue is very difficult if not impossible to remove completely short of surgery. Prevention is better than the cure.

When is PCT needed?

PCT should consist of a SERM due to its ability to stimulate LH (and therefore Test) production. PCT is only necessary when using AAS. At this point in time, all SARMs have little to no impact on LH.

When is an AI needed?

SARMs do not aromatise. Therefore an AI is not necessary on a SARM only cycle. AAS do aromatise. An AI is usuallynecessary on a cycle that includes AAS but is highly individual and takes time and effort to dial in via blood tests. There are some exceptions such as EQ & mast which do not aromatize, and do not require an AI by themselves and even have some AI value, and tren which does not aromatise but will instead elevate prolactin. These are the exceptions, not the rule.


AI is a must for most AAS cycles, followed by a PCT using a SERM if not cruising. AI is not necessary on SARM only cycle. SERM use on a SARM only cycle would be experimental only, and is not a best practice.

Most common SERMs & AIs for /r/PEDs purposes and their approximate doses:

Remember, doses can be highly individual and may take time to dial-in.