Birth control pills are a form of hormonal contraception. Many women use the pill to prevent pregnancy. If you use it perfectly, the pill is 99% effective in preventing pregnancy. However people can easily forget or miss pills and are only 91% effective with "typical use". The pill can also be used to reduce heavy periods, treat acne, and ease symptons of certain reproductive system issues.

What are the different kinds of birth control pills?

There are two main kinds of birth control pills — combination birth control pills, which contain estrogen and progestin, and the minipill, which contains only progestin.

Birth control pills are packaged in different ways:

Conventional. Conventional packs usually contain 21 active pills and seven inactive pills, or 24 active pills and four inactive pills. Bleeding occurs every month when you take the inactive pills.

Continuous dosing or extended cycle. These packs typically contain 84 active pills and seven inactive pills. Bleeding generally occurs only four times a year during the time when you take the inactive pills. Formulations containing only active pills — eliminating bleeding — also are available.

Combination birth control pills are also categorized according to whether the dose of hormones stays the same or varies:

Monophasic. In this type of combination birth control pill, each active pill contains the same amount of estrogen and progestin.

Multiphasic. In this type of combination birth control pill, the amounts of hormones in active pills vary.

Combination birth control pills that contain less than 50 micrograms of ethinyl estradiol, a kind of estrogen, are known as low-dose pills. Women who are sensitive to hormones may benefit from taking a lower dose pill. However, low-dose pills may result in more breakthrough bleeding — bleeding or spotting between periods — than do higher dose pills.

The minipill doesn't offer as many choices. There's a single mixture and formulation, and all the pills in each pack are active.

How do the different birth control pills work?

Combination birth control pills suppress ovulation — keeping your ovaries from releasing an egg. Combination birth control pills also thicken cervical mucus and thin the lining of the uterus (endometrium) to keep sperm from joining the egg.

The minipill thickens cervical mucus and thins the endometrium — preventing sperm from reaching the egg. The minipill also sometimes suppresses ovulation. Unlike combination birth control pills, the minipill doesn't contain estrogen. The progestin dose in a minipill is also lower than the progestin dose in any combination oral contraceptive pill.

What are the cons of combination pills?

  • No protection against sexually transmitted infections, including HIV
  • Increased risk of heart attack and stroke
  • Increased risk of blood clots, with a slightly greater risk of blood clots linked to certain types of progestin used in the pill (though the overall effect of progestin type on blood clot risk is small)
  • Side effects such as irregular bleeding, bloating, breast tenderness, nausea, depression, weight gain and headache

What are the cons of the minipill?

  • No protection against sexually transmitted infections, including HIV
  • Potentially less effective than combination birth control pills
  • Must be taken at the same time every day (taking the minipill more than three hours later than usual means backup birth control will be needed for at least two days)
  • Side effects such as irregular menstrual bleeding, ovarian cysts, decreased libido, headache, breast tenderness, acne, weight gain, depression and hirsutism
  • Slightly increased risk that if pregnancy occurs it will be ectopic — the fertilized egg implants outside the uterus.