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IF you had regular period during previous times and missed it for whole month now then you must consult a gynecologist to rule out pregnancy- get serum hCG
levels and an ultrasound
The drugs provera
both contain synthetic progesterone
and are contraindicated if you happen to be pregnant.
If you are sure that you are not pregnant then you can take either of them, side effect profile is similar.
Take the medicine under the supervision of a physician.
If you have any further query please let me know I would try to answer it for you.
Wish you a great health!