What should a female اعتكاف participant with cancer do if she fears taking pills to delay her menstrual period, especially since she intended to perform اعتکاف before her recovery?

It is permissible to take pills to prevent menstruation for the purpose of performing acts of worship such as Umrah and Hajj, provided that they do not harm the body. Therefore, a doctor must be consulted before taking these pills, to ensure that they do not conflict with treatment or harm one's health, as a Muslim is commanded to preserve their body. If the pills are harmful, it is not permissible to take them. In such a case, one can begin I'tikaf (seclusion in the mosque) and interrupt it when menstruation begins, because it is not permissible for a menstruating woman to remain in the mosque. However, if the pills are not harmful, there is no objection to taking them.