Aromatase Inhibitors (AIs) vs. Selective Estrogen Receptor Modulators (SERMs)

Anyone who wants to use anabolic steroids as performance enhancers should take the time to learn about the various products and supplements that will prevent unwanted side effects.  

Aromatase inhibitors (AIs) and selective estrogen receptor modulators (SERMs) are two very popular – and very important – parts of an anabolic steroid cycle. Understanding the difference between the two is vital to maintaining your health and your physique.

What Do AIs and SERMs Do in the Body?

Just as AIs and SERMs are very different, they are also very much the same. Both of these compounds provide ways to control the amount of Estrogen in the body, which is vital for preventing some significant side effects.

Many of the most popular Anabolic steroids convert to estrogen, and when too much estrogen is present, you may notice some of the following:

  • Decreased libido
  • Significant bloating
  • Gynecomastia (a condition in which men develop female breast tissue)
  • Weight gain
  • Irritability
  • Impotency

AIs and SERMs both prevent these side effects, but they do so in very different ways.Aromatase Inhibitors

What Is an AI?

An AI, or aromatase inhibitor, prevents a steroid from converting to Estrogen. Androgen is common in anabolic steroids, and some are more androgenic than others are. 

Once these androgens are in the body, they turn into estrogen over time with the help of an enzyme known as aromatase. An aromatase inhibitor binds to the aromatase enzyme, thereby making it ineffective at converting androgens into estrogen.

However, it takes time for AIs to build up in the bloodstream. One of the most popular questions is “What is the best aromatase inhibitor?” but the answer varies depending on the unique situation. There are three AIs that are incredibly popular among bodybuilders.

Aromasin – Some men prefer Aromasin over the other two AIs on this list for one important reason: it’s a suicidal inhibitor, which means that once it’s bound to the aromatase, it remains bound until the estrogen and the enzyme are gone. It seems to work better than the other two for some men, but you must come off of it carefully; if you still carry steroid metabolites in your system when you stop using Aromasin, the symptoms can rebound.

Arimidex – Arimidex is arguably the best aromatase inhibitor Bodybuilding enthusiasts can buy. It’s incredibly mild when compared to Aromasin and Letrozole, and because it is competitive rather than suicidal in nature, it simply competes with the estrogen for the aromatase enzyme. This allows for some estrogen to make its way into the bloodstream, which is ideal.

Letrozole – Finally, letrozole is the most powerful of the three aromatase inhibitor options listed here. It is detrimental to your cholesterol levels when used at doses of more than 2.5mg each day, and it can also create some unwanted side effects. However, it is the go-to AI for men who have begun to experience the symptoms of Gynecomastia – it is the only AI that can reverse it in its early stages.

What Is a SERM?

On the other hand, a SERM, or a selective estrogen receptor modulator, binds to the estrogen receptor itself rather than the aromatase. While a SERM blocks some of the actions of estrogen, it allows others. Even in the male body, some estrogen is important.

While an AI inhibits all estrogen, a SERM just prevents its negative effects by blocking estrogen’s androgenic properties. A SERM plays another important role in a bodybuilder or athlete’s performance enhancement regimen, too.

Not only does it help to block some of the estrogenic effects of steroids, but it also stimulates the production of two very important hormones in the male body – luteinizing hormone (LH) and follicle-stimulating hormone (FSH) which are required for natural testosterone production.

Anabolic steroids will bring that production to a grinding halt; when exogenous hormones fill the Testosterone receptors, there’s no need for your body to make more. Unfortunately, once you stop using steroids, your body won’t immediately start making testosterone again, and you’ll need to proactively stimulate that production.

SERMs trigger the production of the hormones that essentially tell your testicles to produce testosterone, and that’s why a SERM is a vital part of your PCT. Two of the most popular SERMs in terms of performance enhancement are Nolvadex and Clomid, and while they are very similar in terms of molecular makeup, most users agree that there are some notable differences between the two.

Nolvadex – The stronger of the two SERM options, Nolvadex can decrease estrogen better milligram-for-milligram than Clomid. It’s also been shown to be safer since it can improve lipid profiles and reduce estrogen levels without a host of unwanted side effects. However, there is also evidence to suggest that Nolvadex may have more of an impact on your gains than Clomid, which means you may end up losing a couple more pounds of mass if you choose this product.

Clomid – Clomid is weaker than Nolvadex per milligram, but when used at the right dosage, it can provide just as much protection against estrogenic effects, and it can boost Testosterone levels just as effectively. Because it is not as potent as Nolvadex, you won’t lose as much of your gains.

The choice is yours, so it’s important to consider the risks and the benefits of using one over the other. SERMs are usually reserved for post-cycle therapy, though there are some men who use them successfully on-cycle in the place of an AI to block estrogen. The chart below shows an average PCT cycle for each SERM.

SERMWeeks 1-2Weeks 3-4Weeks 5-6 (Optional)

Everyone should use either Clomid or Nolvadex post-cycle for at least four weeks. This is often sufficient to restart testosterone production and help balance the gap between testosterone and estrogen in the meantime. Men who are especially sensitive may need to use a SERM for another one to two weeks beyond this; other men may find that four weeks is plenty.

Aromatase Inhibitors vs. Selective Estrogen Receptor Modulators

Which Is the Best Choice?

While an AI blocks estrogen production and may ward off side effects, bear in mind that the body continues to send chemical signals that it needs more estrogen. 

As such, the body continues to produce more estrogen, resulting in an endless cycle and sometimes a “rebound” effect in which estrogen levels suddenly spike after stopping the AI. A SERM only affects the way the body uses estrogen, and as far as the body knows, there is plenty of estrogen to go around.

Thus, while an AI can prevent unwanted side effects during anabolic steroid cycles, it is imperative to use a SERM during post-cycle therapy to combat rebounds and bring levels back to normal. Both AIs and SERMs play important roles for people who choose to enhance their performance with anabolic steroids.

However, knowing which product to use in which situation can make all of the difference when it comes to preventing potentially serious side effects.

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