Is Regestrone Safe To Take For Period Regulation?

I had brown discharge for 3 days after 4 days of my expected period and it was not normal period kind of bleeding what i used to get, so i consulted my doctor and she has asked me to take Regestrone 50mg sandoz tablet twice a day for 21 days. Could you please let me know is it safe to take it? And my periods were regular before and this is the first time i am having this problem and i am not sure whether the 3 days mix of red and brown color discharge was my regular period or not? And is it safe to take the tablet? as this is the first time i am having this problem.