Tamoxifen Dosage is an antibiotic often used to treat breast cancer. It works by inhibiting the actions of estrogen in the body, slowing or stopping the proliferation of cancer cells. Tamoxifen dose for breast cancer varies according to the individual and stage of the cancer. Typically, the suggested amount is 20mg to 40mg taken once daily. Tamoxifen is normally prescribed for 5 to 10 years, however, this can vary depending on the individual and their response to the treatment. Even if you begin to feel better, it is critical to continue taking tamoxifen for the full duration of the prescription.