I am 21 years and dont have periods for 3 months what medicine should i have for irregular periods?

The question is more complicated than it might appear. If you are on the birth control pill this is not abnormal, and, if you are not, it still may be normal but there are some basic tests that should be done if not done already. If those are normal you could take a progesterone periodically to bring a period, but anything you do should be done in consultation with your doctor.

A three month gap between "periods" may be due to pregnancy, or to absence of ovulation. Obtain a serum HCG test , which is very accurate for pregnancy, and if negative, see your GYN physician for an exam and other testing, as polycystic ovarian syndrome (PCOS) can present with absent menstruation (due to lack of ovulation).