To start taking the pill, you must get a prescription from a doctor, who will decide which type and dose is best for you. After getting the prescription filled, a woman should begin taking the pill according to her doctor's instructions. For maximum efficacy, the pill should be taken at the exact same time every day. Combination pills come in 21 or 28-day packs. Both have 21 hormonally active days of pills, while the last seven days in a 28 pack are simply "reminder" pills, which do NOT contain any hormones. Progestin-only pills come in 28-day packs, every day of which contains hormones. Regardless of the type of combination pill a woman takes, menstruation occurs during the fourth week of her cycle. Women taking combination pills have the option of suppressing a period by skipping the "placebo" week and starting a new pack immediately. Meanwhile, with the progestin-only pill, the periods can be irregular, or even disappear altogether. The pill is such a popular method of birth control due to its price tag-about $15 U.S. dollars a month-and its high rate of effectiveness-over 99 percent when used perfectly.