There is no single “safest medicine” to make periods come late or to make them regular every month without knowing the reason. Do not start hormone tablets such as norethisterone, medroxyprogesterone, or birth-control pills on your own, especially for a teenager. They can be unsafe in pregnancy and may hide an underlying problem.

First step: if there has been any sexual contact, do a home urine pregnancy test, even if contraception was used. Pregnancy is the first cause that should be excluded when a period is late.

A late period can happen from stress, weight change, intense exercise, poor eating, PCOS, thyroid problems, some medicines, or normal irregular ovulation in the first 1-2 years after periods begin.

For adolescents, a clinician may check pregnancy, thyroid and hormone levels, and assess for PCOS or nutrition-related causes before deciding whether hormonal treatment is appropriate. The ACOG guidance on adolescent cycles recommends evaluation when more than 3 months pass between periods. Their amenorrhea information also advises assessment when periods stop for over 3 months without explanation.

If the period is only a few days or one cycle late and pregnancy is ruled out, it is usually better to wait and track the cycle rather than take a medicine.

To delay a period for a trip, wedding, exam, etc., a clinician may prescribe norethisterone. It needs to be started at least 3 days before your period is expected. A common prescribed regimen is 5 mg three times a day, and bleeding usually starts within about 3 days after stopping. It is for short-term occasional use, not a regular solution. The NHS Specialist Pharmacy Service guidance gives this approach.

Do not take it without a pharmacist or doctor checking that it is safe for you. Do not use it if:

It is not birth control. Use condoms or your usual reliable contraception to prevent pregnancy. The official medicine leaflet specifically says not to take norethisterone in pregnancy and notes that it is not contraceptive.

If your period is due in less than 3 days, has already started, or you are under 18, speak with a pharmacist, gynecologist, or doctor first. Seek urgent care for chest pain, shortness of breath, coughing blood, or one-sided leg pain/swelling while using hormonal medication.