@Chevvy - how old are you? Have you gone thru menopause?
My surgeon told me that I'd go onto Tamoxifen, so I was expecting to - but it was never even mentioned by my Onc.
Back in it's day, Tamoxifen was the ONLY AI med available (back when the only surgery was radical mastectomies - irrelevant of tumour size) & both my brother's Mothers in Law were on it 30+ years ago for 10 years and they are are still here today, aged in their 90s! So I reckon that proves it is an effective med!
These days, I believe Tamoxifen is used more for Pre Menopause women - tho depending on the type & grade of the tumour, some still go onto the other AIs, eg Letrozole etc.
I went straight onto Letrozole - I am 20 years post menopause. Then Exemestane, and have now 'settled' on Arimidex for the last 2.5 years!! So it is NOT unusual to 'have a change' mid stream either.
It is a shame that this Dr (is it a GP) has now undermined your confidence in your medical team - I am guessing it isn't the Onc who prescribed you the meds in the first place? Keep in mind that GPs are NOT Oncs!! They may 'think' they have a broad understanding of EVERYTHING ..... but they do not - specially in the case of cancer treatments.
That's why Oncs are 'specialists' and GPs are 'General Practitioners'!! They have a 'general knowledge' of lots of stuff .....
SO .... Definitely get a second opinion (and put it in writing to your original Onc as well, to get HIS explanation as to why he put you onto Tamoxifen), and make sure you record any meeting on your phone for future reference - write down any questions that you have regarding which AI you are on & WHY ...... Make sure you give THEM a printed copy of the questions (and make notes on your own copy ....) & don't leave til all the questions have been answered.
All the best xxx