Does breaking the fast during menstruation – resulting from taking medication to induce menstruation in Ramadan due to irregular periods and the difficulty of fasting – necessitate sin, expiation, or merely making up the days?

If taking medicine is intended for breaking the fast in Ramadan, then it is not permissible. You must seek forgiveness and not repeat the action. You only need to make up the days you missed fasting, and no expiation is required.