FK506, known as Tacrolimus, is an immunosuppressive drug that inhibits lymphocytes, especially B cells, and has been used to treat immune rejection, for example, after organ transplantation. In addition, Tacrolimus can also be used to treat systemic lupus erythematosus. Overall, the drug is relatively safe and can be used by women who are pregnant or preparing for pregnancy, with no effect on the fetus. In conclusion, the drug should be taken under the guidance of a rheumatologist and reviewed regularly.