Competitive aromatase inhibitors, such as Cytadren, Arimidex, and probably Proviron, bind to the same binding site on the aromatase enzyme that testosterone does. By doing this, they allow less testosterone to bind to aromatase. So, less testosterone is converted to estradiol (estrogen).

Here�s an important thing: the effectiveness of competitive inhibitors decreases as the amount of the normal substrate increases. Suppose that you had equal amounts of inhibitor and normal substrate in the blood, and they bound to the enzyme equally well. Then the inhibitor would at any moment be taking up half the sites that the normal substrate otherwise would, so it would reduce conversion rate by 50%. But if the amount of substrate is increased 10 times while the amount of inhibitor remains the same, then the inhibitor would be outcompeted by the more numerous substrate molecules. It would therefore be rather ineffective.

For example, with more testosterone molecules available, and similar binding strengths, the enzyme will mostly bind testosterone. It will then mostly be working to produce estrogen. To obtain the 50% reduction we had before, then the amount of inhibitor would also have to be increased 10 times.

To be really effective, the inhibitor must either be present in higher concentration than the normal substrate, or must bind more tightly.

With Cytadren or Proviron, it takes quite a lot of inhibitor to outcompete high testosterone levels. With Arimidex, rather little, even 1 mg/day, can be sufficient because it binds so strongly.
triphenylethylenes, and are not steroids. The differences are relatively minor, but seem to affect an important characteristic of these compounds: drug metabolism.
The other general approach is estrogen receptor antagonism. If a molecule binds strongly to a hormone receptor, but does not activate that receptor and makes it unresponsive to the normal hormone, then it is a receptor antagonist. Clomid (clomiphene) and Nolvadex (tamoxifen) follow this approach. These drugs are very similar structurally. They are both what are called

Both tamoxifen and clomiphene are metabolized to other related compounds which can be estrogenic or anti-estrogenic. Both act as estrogens in bone tissue, perhaps after metabolism, which is a very useful property for female patients, for whom these drugs are usually intended. (Otherwise, an anti-estrogen could lead to osteoporosis.) Tamoxifen seems particularly prone to acting as an estrogen in the liver, which may account for reduced IGF-1 levels seen when this drug is taken.Users generally seem to agree that when tamoxifen is used, gains are a little less than what otherwise would be expected. (Let�s not take this too far though: many people have made great gains while using tamoxifen as an anti-estrogen. And it�s always hard to say what "would" have been the case if a drug had not been included.) I�ve heard nothing but good about clomiphene, though.

Proviron, an anabolic steroid, is particularly interesting. I suspect that it not only acts as an antiaromatase but in an unknown DHT-like anti-estrogenic manner. This might involve estrogen receptor downregulation for example. In any case, aromatase inhibition and/or Clomid don�t seem to give the same effect on appearance and muscle hardness as when Proviron is included.

How much of these agents is needed for effective estrogen suppression?

Again, it depends on the dose of anabolic/androgenic steroids (AAS) and it depends what type of AAS is being used.

With Primobolan or trenbolone there is no need for these drugs.

With nandrolone, an aromatase inhibitor will be of no use, because aromatase is not used in the aromatization of nandrolone. A rather small amount of estrogen receptor antagonist can be useful. 12.5 to 25 mg Clomid would be plenty for 400 mg/week Deca.

With testosterone, stacking of an aromatase inhibitor and an estrogen receptor antagonist will give the best results. Cytadren use should not exceed 250 mg/day in my opinion. This alone would not be sufficient for say 1 g/week or more of testosterone. With such a dose, ideally one would add in 50 mg/day Clomid. Proviron at 100 mg/day could substitute for the Cytadren. Or Cytadren and Proviron can be used in combination, 125/50 or higher, together with 50 mg/day Clomid.

For lower doses of testosterone, proportionally less antiestrogens can be used.

Arimidex is very effective but extremely expensive. 1 mg/day of this is at least as effective as 250 mg/day Cytadren. If a milligram per day cannot be afforded, use of half a milligram would allow Cytadren use to be cut in half, which may be desirable.

How does Clomid "stimulate" testosterone production at the end of the cycle?

It really doesn�t. Rather, by acting as an estrogen receptor antagonist, it reduces the inhibition that results from elevated estradiol levels. This helps return LH to normal levels, which helps testosterone to return to normal levels (if the testicles have not atrophied).

How does hCG help?

Acts as an LH receptor agonist, thus substituing for LH. It does nothing to help the hypothalamus and pituitary. Thus, it can be effective during the cycle to help avoid testicular atrophy, but is not best used in the taper when one is attempting to restore LH production. Increases in natural testosterone, stimulated by the hCG, will act to inhibit LH production. Thus, you can see where hCG use is counterproductive in the taper itself.

Can Clomid, taken throughout a cycle, completely eliminate inhibition?

I do not believe so. There is also androgenic inhibition mediated by the androgen receptor, which has nothing to do with the estrogen receptor. Androgenic inhibition is unavoidable and cannot be helped by estrogen receptor antagonists. However, use of Clomid throughout a cycle can definitely reduce the degree of the inhibition and allow a speedier recovery at the end of the cycle.

Is it safe to take Clomid for so many weeks? I heard it should only be taken for 2 weeks.

The two week idea comes from the fact that medically its main use is to help women with fertility problems. Because of the menstrual cycle, there are only certain times of the month when there is any chance of ovulation. It is pointless, then, for these women to take the drug for more than two weeks at a time. Some have misconstrued this to apply to males.

Men have taken the drug in clinical studies for a year continuously. It is a rather safe drug.

Why do you say not to use more than 250 mg/day of Cytadren?

Cytadren has two main therapeutic activities. At high doses, such as a gram per day, it is a very effective inhibitor of the enzyme desmolase, which is required for all steroid production, and is rate limiting for the production of cortisol. So the drug is very useful for treating patients with Cushing�s Syndrome, who produce abnormally high levels of cortisol.

It is also an inhibitor of aromatase, and it is a better aromatase inhibitor than a desmolase inhibitor. About 250 mg/day is sufficient for fairly good inhibition of aromatase, resulting in only fairly low levels of desmolase inhibition.

As dosage increases, aromatase inhibition does not improve much, but desmolase inhibition increases greatly.

Even at 250 mg day, there is still significant desmolase inhibition. Other side effects, such as lethargy, may bother some individuals even at this dose.

Why is desmolase inhibition bad? I have read that cortisol is the enemy of our muscles, and we want to reduce it.

Those articles are written by people trying to sell you alleged cortisol-reducing supplements.

While abnormally high levels of cortisol are indeed muscle wasting, abnormally low levels of cortisol do not result in extra muscle growth, and cause joint problems.

You�ve talked about tapering off Cytadren. Why?

There is a feedback mechanism for production of cortisol. Low levels of cortisol enhance release of corticotropin releasing hormone from the hypothalamus, and ACTH from the pituitary. Both will result in higher production of cortisol.

So moderate inhibition of desmolase will temporarily reduce cortisol, but soon it will be back to normal as this feedback mechanism compensates.

If you then suddenly discontinue the drug, then these elevated ACTH levels will result in abnormally high cortisol for a time, until the body adjusts again. This can be avoided simply by tapering down over about a week.

Should Cytadren be taken all at once, or in divided doses?

Because the half life is only 6 or 8 hours, if the drug is taken only once, then through part of the day there will be little drug in the system, and little anti-aromatase activity.

The rest of the article was a good read.