Hi, I have pcos, my doctor asked me to take medicine on fifth day of my periods, I forgot it can I take on sixth day of my periods?

Yes you can. The timing for ocp initiation in women with pcos is not critical as menses are seldom regular or of ovulatory type. Taking ocp will help in suppressing your androgen levels which in turn will promote ovulation and regular menses. Good luck.
Yes. I think the medication is Clomid (clomiphene) or femara. You may take either of these medication on cycle day 6. If that is the case, i recommend you to take for 4 days. Good luck!
If it is ocp. I'm not sure what your doctor prescribed, but if you're on oral contraceptive agents then it would be ok to start on day #6. It is very important to remember to take ocp's daily. Please check with your physician also.