The Qanun was translated into Latin as Canon medicinae by Gerard of Cremona . (Confusingly there appear to have been two men called Gerard of Cremona, both translators of Arabic texts into Latin. Ostler states that it was the later of these, also known as Gerard de Sabloneta, who translated the Qanun (and other medical works) into Latin in the 13th century.)  The encyclopaedic content, systematic arrangement, and combination of Galen's medicine with Aristotle's science and philosophy helped the Canon enter European scholastic medicine. Medical scholars started to use the Canon in the 13th century, while university courses implemented the text from the 14th century onwards.  The Canon ' s influence declined in the 16th century as a result of humanists' preference in medicine for ancient Greek and Roman authorities over Arabic authorities, although others defended Avicenna's innovations beyond the original classical texts. It fell out of favour in university syllabi, although it was still being taught as background literature as late as 1715 in Padua.  
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.