I started on 20 mg tamoxifen, by 3 months had very low motivation, brain fog? depression? Stuck it out for another 3 months but by then was having headaches 24/7. Other side effects not so bad, some warm flashes and difficulty cooling off after overheating in weather, some moodiness.
I was postmenopausal 69 at that time, DCIS, surgery, radiation.
I dropped to 5 mg and it was like night and day. Motivation quickly returned - I was so grateful, I signed up for guitar lessons and started TaiChi. No headaches.

Important note! I’ve found my regular doctors (general practice, endocrinologist, gynecologist) defer to the oncologist and will not question the medication dose or suggest that it might be causing my issues. They say, cancer treatment comes first. Yes. But. If it means going on other meds then I think they HAVE to be part of the equation to decide priorities.
When I saw my general MD about the headaches, she went through the possibilities for causation, testing everything that might cause headaches including a brain MRI and could find nothing. She said I should tell the oncologist of her findings. The oncologist, and the cancer neurologist who reviewed the MRI, said headaches were most likely from tamoxifen and my dose was lowered.
The problem with depression, or brain fog that I had, is its much more difficult to pinpoint a cause. Often drugs are used because they have nothing else that works. I was told I could try an anti-depressant.

About AI or tam:
Oncologist said I had a choice of tamoxifen or AI. AI’s do show a better response, but when you read the research the “significant” number is very small.

I’ve been on tamoxifen for 3.5 years. Was told by survivorship doctor that in Dec 2026 when the San Antonio Breast Cancer Symposium research has been presented, protocols will probably change, it will most likely be recommended that I can stop tamoxifen. Latest research is showing that for the cancer I had, DCIS ER/PR+ luminal A, three years has been shown to be as effective as five.

Also, the latest research is showing that while 20 mg of tamoxifen is preferred dose, 5 mg provides the same results for postmenopausal women with certain cancers, with less side effects.