PIP: When taken regularly, oral contraceptives (OCs) are 99% effective in preventing pregnancy. This information sheet identifies the difference between the 28-day or 21-day packet of pills and reviews how OCs work and how they affect the menstrual cycle, what do do if one misses a pill, side effects, and ensuring maximum protection. The only difference between the 28-day and the 21-day packet of pills is the 7 inactive pills in the 28-day packet. With the 28-day pack, one starts the packet on the 1st day of menstrual bleeding, taking the pill from the shaded section marked with the appropriate day. One continues to take the pills every day, in the direction indicated by the arrows. The next period will occur when one returns to the inactive pill section. With the 21-day packet, the 1st packet is started on the 5th day of menstrual bleeding, whether or not the bleeding has stopped. One's menstrual period will occur 2-4 days after finishing the last packet. The pill prevents pregnancy by changing the hormone balance in the body to stop ovulation. Menstrual periods tend to become shorter and lighter. If a pill is missed the forgotten pill must be taken as soon as possible, even if it means taking 2 pills in 1 day. The most common side effects include nausea, breast tenderness, and breakthrough spotting. More serious side effects include migraine headaches and high blood pressure. Diarrhea and vomiting may reduce the absorption of the pill; to ensure maximum protection in those cases, one should keep taking the course of the pill but use additional contraception until the end of the packet.