Is Duphaston Suggested For Irregular Periods?

Hi, I am 28 yrs old, 5'5, 150lbs. I have irregular periods, approximately 30-40 day cycles. I am NOT trying to get pregnant NOR would I like to conceive as of this time. My doctor prescribed Duphaston to help me get my period as I have irregular periods. But I am worried because this might make me more fertile. Will Duphaston help me get my period if I am not yet pregnant as of this time? Because I do not want to get pregnant, should I refrain from all sexual activity while on Duphaston and for how long? Thanks.