Tamoxifen 10mg vs 20mg

I was on Anastrozole for 6 months my doctor switched me to Tamoxifen because of severe muscle pain. I was given 20mg of Tamoxifen is that the regular dosage every day. It also comes in 10mg. I saw a study that women were given 5mg each day by cutting the 10mg in half and was wondering if anyone has cut their dosage so it is better tolerated.