If you don’t want to read the whole thing - just highlighting experiencing severe fatigue when on tamoxifen AND finding out it was making little difference anyway. Ask for info about why they think you should be on it and don’t tolerate unbearable side effects. Push to speak to the oncologist if you’re not ok.
I had a lumpectomy a year ago this week and started on tamoxifen in November 2025. I didn’t experience any side effects initially and was feeling well by the time I started radiotherapy in Jan 2026. I slept a lot in the 4 weeks after radiotherapy but then started to feel much better - coping ok with a house move etc. then at the end of February I had a huge setback energy wise. I thought it was because I had overdone exercise . I had no energy at all, terrible brain fog and felt sick most of the time. I couldn’t work and was in bed a lot of the time.
After a couple of weeks I asked the BCN who was assigned to me if it might be the tamoxifen . She said no it was probably radiotherapy fatigue and it could last for years 
I felt pretty desperate then.
A week later I contacted her again and asked for an appointment with my oncologist to discuss stopping tamoxifen in case it was causing me to be so unwell.
I got a telephone appointment 7 weeks later. During those 7 weeks I also started to experience heart problems - a severely elevated heart rate all the time and missed beats and some tachycardia. I had one A&E visit because of it , a 24 hr trace and a referral to the cardiologist.
when I finally spoke to my oncologist she told me that tamoxifen causes ‘severe and debilitating fatigue’ for some women. I asked her how much of a difference tamoxifen was making to my 10 yr mortality risks and it was only 1-2%.
I stopped taking Tamoxifen straight away. The plan was for an 8 week treatment break. I was told it could take 3 months for the drug to fully clear out. After 48 hrs I began to feel a bit better. The improvement continued and within a month I was physically well again: by the 3 months my mind felt fully clear again. I spoke with my oncologist after 8 weeks and we agreed it was not a drug I needed to take.
I’m posting because if I’d known it was making such a small difference to my future risk I would have stopped it much sooner . I’m frustrated (and angry) that side effects are downplayed and that it’s not taken seriously when a drug is really stopping you from having much quality of life. I discovered as well that it’s a known thing ( there are research papers about it ) that for some women tamoxifen effects worsen at about 4 months, as mine did.
Everyone’s situation and risks are different but I think it’s worth knowing what tamoxifen is supposed to be doing for you so that you can properly assess if it’s worth taking. Hope this helped someone.