Thanks for your query.Secnidazole
is a anti protozoal and has few side effects like Nausea,metallic taste
, urticaria, rashes etc but it has no effect on menstrual cycle.
As you have missed periods it could be due to some other reason like pregnancy,hormonal imbalance,polycystic ovary syndrome
,thyroid disturbances,severe mental stress.
I would suggest you to consult a Gynaecologist and get examined.
After proper history and examination she will prescribe the necessary medicine to regularize your periods.