It is very interesting how insanely complex the human body is. For an example, I will use a typical time that the body goes into a catabolic state: nighttime/morning. Often times people will finish their dinner around mid-evening, let’s say 7pm and will maybe have some junk snack such as a bowl of ice cream and then hit the sack. Due to not having any protein since dinner [if you even had any then], then the ice cream not lasting very long as energy in your body, you will go to bed & a couple hours into sleeping, your body will actually run out of energy. Once it runs out of energy to burn, it may tap into fat cells, but shortly after will begin panicking that it may be getting starved and will tap into what it feels is a much better source of energy: muscle. [You should consume casein protein to combat this.]
It should be noted that in theory if one was to consistently suppress your natural estrogen levels for a long period of time, this would negatively impact your health, including your cholesterol. Due to the ability of Letrozole- to inhibit estrogen so much, this should definitely be a concern to most users. However the research that has focused on the relationship between use of letrozole and cholesterol levels is rather inconsistent in it's findings. Many studies have concluded that the compound is detrimental to both a user's HDL and LDL cholesterol levels, while other research has found no link. Obviously individuals are best served to monitor their cholesterol while using any compound via blood tests however barring that, letrozole should simply not be run for extended periods of time if at all possible. Doing so could cause serious medical complications.
Along with the issues related to blood lipids is the fact that many users complain that their libido is dramatically reduced when using the compound. This is related to the fact that estrogen is partly responsible for the regulation of an individual's sex drive. Since Letrozole- is so potent it can often drive estrogen levels too low and this inhibits a user's libido. To avoid this users can lower dosages, but some anecdotally report that even extremely low doses of the drug can cause problems. If this is the case a less potent compound such as exemestane or anastrozole may be a more appropriate option.