The 5mg tamoxifen is said to be very tolerable with the same efficacy as the original dose of 20mg.

My experience with tamoxifen is that the first couple of weeks I felt changes but it got worse by week three, but still pretty mild at this stage. By 3 months I started feeling more side effects that seemed to level off. I felt that some of it, minor cramping and the inability to regulate body heat just became part of daily life and not too disruptive. I didn’t really have hot flashes, but if my body was overheated, like a hot day in the car or a hot bath, my body couldn’t cool off fast enough. I’d be uncomfortably hot for a while, but didn’t have that flash of heat. I also had brain fog, which I think I adjusted to and didn’t realize how foggy I was until I did a drug holiday and then lowered the dose. A drug holiday, and subsequent brain MRI, was recommended because I was having headaches 24/7 that had no other cause.

I started on 20 mg for 6 months. I self prescribed 10 mg because I couldn’t stand the headaches at 20 mg - and couldn’t get in to see the doctor fast enough. I did not notice a change in symptoms with 10 mg.
Then did a recommended drug holiday, I did 5 weeks. And started on 5 mg. So I went through that initial start up a couple of times and it was the same on both, 3 week and 3 month markers were when I felt the biggest changes.
I have self prescribed myself a lower than 5 mg dose (I’m not recommending this to anyone because there hasn’t been enough research to show that 2.5 mg is enough, although DeCensi’s research showed that it might be). I was having memory issues - and it’s hard to say if it’s just old age or tamoxifen - but it is concerning to me, and I thought if I lower the dose a little and regain my memory then that’s the answer. I’m sure my oncologist cannot ethically recommend the 2.5-3 mg dose I’m taking because, ethically, he can only recommend what he has data to show it works.