Letrozole is Novartis’ entry
into the breast cancer treatment world.
It’s a Type-II Aromatase Inhibitor (AI), which means that it
competitively binds to the aromatase enzyme and inhibits the enzyme’s
ability to metabolize testosterone into estrogen. This drug was
developed to fight breast cancer by inhibiting the aromatization.
Letrozole is probably the most powerful Aromatase Inhibitor
athletes today. It has been shown to reduce estrogen levels in women
with breast cancer by 98% or more 
SO clearly, it’s useful for
administration to male steroid using athletes who are eager to prevent
some of estrogen’s nastier effects on their bodies- development of
breast tissue, water retention, etc…
When we take a look at its effects in men, Letrozole actually reduced
estrogen in one test subject to undetectable levels 
clinical study, intravenous administration of Letrozole (2.5mcg for 28
days), Letrozole lowered Estrogen by 46% in the young men tested, and
62% in the elderly subjects. In addition, Letrozole also significantly
increased LH levels to a whopping 339 and 323% in the young and the
elderly, respectively and Testosterone by 146 and 99%, respectively. 
Letrozole was also
able to produce a peak LH response to
Gonadatropin Releasing Hormone
equal to a 152 and
52% increase from
baseline in either young or older men, respectively.
As you can see, Letrozole is a very powerful drug, and as a result,
only very tiny doses are necessary. An effective daily dose of
Letrozole for most people is usually about .25-.5mg/day, even though
clinically, it is typically used at 2.5mgs/day. Twenty micrograms of
Letro was enough, in one study done on men, to reduce estrogen levels
by almost a third. 
effects on cholesterol are, really difficult to pin down precisely.
They are, in the words of one researcher: "inconsistent.” I can tell
you that in my opinion, reducing your bodies estrogen to virtually
nothing, will eventually take its toll on your cholesterol profile, and
will kill your sex drive and your joints- all of which require estrogen
to function safely and effectively.
Even if you take very low
doses of Letrozole, it will build up to reasonable blood plasma levels,
as it has a 2-4 day half-life, and this long half life also means you
need to take Letrozole for 60 days to get a steady blood plasma level 
, and that it
will take a very long time to clear out of your system.
Letrozole is the only pharmacological “cure” for gyno that I know of to
have ever worked in bodybuilders. In a study conducted on rodents,
Letrozole was able to effectively destroy breast tissue tumors 
it’s also been effective on many bodybuilders who have used it to
eliminate an existing case of gynocomastia. In my case, I used Letro to
get rid of my own gyno, by starting with a dose of 2.5mgs/day and then
lowering it by .25mcgs per week once my symptoms abated.
With regards to using this stuff on a cycle, unless you are extremely
gyno prone, or need to reduce estrogen levels to virtually nothing (for
a bodybuilding contest or whatever), it’s going to be too powerful for
most people. Male and female competitors typically use it to get the
last bits of estrogen related water retention out of them during the
final weeks of contest preparation. But when used on a typical cycle,
Letro is generally overkill unless a ripped look with zero water and
estrogen is desired or if the user is prone to gyno.