Are the extra days in the menstrual period following hormonal medication considered irregular bleeding (istihada) or a new regular cycle?

If the days of menstruation exceed the usual period, they are considered menses as long as they do not exceed 15 days. If they exceed that, it is considered istihadah (irregular bleeding). You cease praying and fasting during your usual menstrual period, then you perform ghusl (ritual bath) and wudu (ablution) for each prayer, and the cause of the blood does not affect this ruling.