Reference: Fataawa Ramadhaan v1 p356 no.309
Majmoo’ Fataawa Ibn Baaz v3 p214
Question: Is it permissible for a woman to take pills to prevent the onset of the menses during Ramadhaan?
Response: There is no harm in that so long as there is a personal interest for the woman [such as] in fasting with the rest of the Muslims and not having to make up [the missed fasts due to menses]; [That said] she must be careful there is no harm in taking these pills because they are harmful to some women.
