I've been on Letrozole, not Anastrazole, but the position is similar. I've been on them since mid 2017, and depending on the pharmacy or the availability of one brand or another, I've personally found no difference. I have other significant health conditions, and am on a total of 14...yes 14 different prescription medications such as prednisolone, beta blockers, blood pressure tablets (2 different ones), blood thinners, proton pump inhibitors, diabetes medications (not insulin) etc and have found the same to be the case across the board. The active ingredient is the same in all brands of that particular medicine, but 'extras' such as binding agents, colourings, coatings etc can differ, and if you have an intolerance to one of these ingredients, then that might make a difference. For example, my adult daughter is very allergic to Tartrazine or E102, a yellow food colouring and has severe asthma attacks if she eats it. Therefore, if she is going to take a tablet which is any colour other than white, she always checks with the pharmacy. Some people are lactose intolerant and lactose is sometimes used as a filler in tablets and so a particular brand which uses it should be avoided.
@AllyJay, you have provided great information regarding the 'extras' in a medication. You are right the active ingredient is same in all brands but may be the extras which are different give us side effects. Thanks so much for providing such a useful information which I have to consider too.
God bless you and your family. with much love