Women must fast in Ramadan just like everybody else does, even though they are on their monthly periods.
According to verse 187 in chapter Baqarah in the Quran, things that break a fast are eating, drinking and sexual intercourse. There are only two types of people who have the permission to not fast and compensate for it later. They are the sick and the travelers. Menstruation neither breaks the fast nor grants a permission to not fast. Opinions that claim otherwise have no basis in the Quran or the Example of the Messenger.
Yet, we know that some women may have extraordinary difficulties in their daily lives during their periods. They may even have to have medication. In such cases, they are considered as sick, and they may fast on some other days after Ramadan.
Another solution for these sick women may be having painkillers injected, provided that the injected medication contains no nutritives but only painkillers. Thus, women can fast the whole Ramadan if they choose to do so.
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