Praise is due to Allah Almighty and peace be upon the Prophet (PBUH), his family, companions and followers;
It is permissible for a Muslim to donate blood during fasting, unless it affects his/her health; indeed, a Muslim shall be summoned if another person is in dire need for blood donation, and his/her fasting shall not be affected. In fact, some jurists compared such matter to cupping, since both processes include drawing out blood; hence, the fast of a blood donor is not affected. Allah Almighty knows best.
