When is it obligatory to perform ghusl after taking menstrual regulation pills, and what is the ruling on seeing yellowish discharge after the cessation of blood, or after ghusl, or after seeing the sign of purity?

It is understood from the Hadith (definition) of Aisha, may Allah be pleased with her, that menstruation is something Allah has decreed for the daughters of Adam. One should submit to it and not use medications to stop it, delay it, or regulate it, except out of necessity, such as for Hajj or to treat istihada (non-menstrual vaginal bleeding).

After using the pills, if the bleeding stops and the woman sees one of the two signs of purity (al-jufuf - dryness, or al-qassah al-bayda' - white discharge), she must perform ghusl (definition) (ritual purification bath), pray, and fast. As for yellow discharge after confirmed purity, it is not considered anything. However, if it appears before dryness or the white discharge, the woman remains under the ruling of a menstruating woman. It is recommended to wait for the white discharge as an indication of the end of menstruation, as long as this does not lead to the prayer time expiring.