I tried generic anastrazole and had trouble with fillers before I even did more than a week. I then tried generic letrozole. I ended up with brand name Femara (letrozole). My doc also said it would be fine to take alternate days, if that helped with side effects, but mine were tolerable and I stuck to daily.

The Femara insert contains a study that showed that 20% of the prescribed dose is effective so it should be okay to do alternate days, considering the half life of the drug and that 20% figure (I am speculating based on their own insert and my doc's approval of alternate days).

Here is more info on letrozole, which probably applies to others.
"Letrozole’s terminal elimination half-life is about 2 days and steady-state plasma concentration after daily 2.5 mg dosing is reached in 2-6 weeks."

So steady state plasma concentration takes awhile and that may explain why some are having more side effects after that period of time. Also, to me, it makes sense that 1) side effects might change over time with a hormonal med and 2) if we stick it out the body MIGHT adjust and stabilize....I have no studies on that but that was my experience and it makes sense.