one blocks production, one blocks receptors

letro does both 99% suicide inhibitor

a few copy pastes

Exemestane is a steroidal suicidal Type 1 irreversible aromatase inhibitor.When exemestane reaches full blood plasma concentration within the blood, it will block up to 98% of estrogen

Nolvadex only blocks estrogen receptors, it allows circulating estrogen to continue to exist. If used for pct of an aromatizing drug, levels of circulating estrogen would be greatly increased when nolva usage was discontinued


Anastrozole is a Type 2 non-steroidal competitive aromatase inhibitor (second generation) which functions by blocking the aromatase enzyme (chromosome P450), the key enzyme responsible for the conversion of testosterone to estrogen.

Letrozole ( third generation) is a non steroidal selective third generation aromatse inhibitor which just like Arimadex will not give off androgenic effects. Its very similar to Arimadex which why they both are called Type2 non-sterodial competitive aromatase inhibitor. The main difference between Femara and Arimadex is that Femara is MORE potent,this will obliterate one’s estrogen by 98-99% within peak concentrations and is detrimental to cholesterol over pronlonged usage

The main difference between Type 1(suicidal) and Type 2(competitive) is that type 1 will deactivate the estrogen and the enzyme will be gone, which means a new aromatase enzyme must be created. In Type 2 AI’s, the AI will compete for the binding site and once the individual stops taking the AI, the effects will come to a hault which can be problematic if you are on or still carry metabolites of a highly aromatizing androgen in your system.


First Aminoglutethimide 250 qid 90.6 MacNeill et al (1992)
Formestane (IM) 250 2w 84.8 Jones et al (1992)
Second 500 2w 91.9
500 w 92.5
Formestane (po) 125 od 72.3 MacNeill et al (1992)
125 bid 70.0
250 od 57.3
Second Rogletimide 200 bid 50.6 MacNeill et al (1992)
400 bid 63.5
800 bid 73.8
Second Fadrozole 1 bid 82.4 Lønning et al (1991)
2 bid 92.6
Third Anastrozole 1 od 96.7 Geisler et al (1996)
1 od 97.3 Geisler et al (2002)a
Third Letrozole 2.5 od >98.9 Dowsett et al (1995)
2.5 od >99.1 Geisler et al (2002)a
Third Exemestane 25 od 97.9 Geisler et al (1998)