Are you taking, or considering taking, a birth control pill? Nearly 12 million U.S. women do. And though you may simply call it "the pill," there are many different types of birth control pills.

Each type of pill has pros and cons. But first, make sure that this form of contraception is right for you.

Here's what to consider.

2 Questions to Ask Yourself First

1. Will you still use a condom to protect against STDs? Male condoms offer the best protection against STDs. With all other methods of contraception, you should also use a condom.
2. How likely are you to forget pills now and then? The answer matters, because it affects how well the pill works. "You should swallow a pill at the same time every day, whether or not you have sex," the FDA's web site states.

When taken correctly, the pill is highly effective at preventing pregnancy. But that phrase "when taken correctly" is key.

About 8% of women who take the pill become pregnant unintentionally each year. In most of those cases, the women forgot to take pills. Yet when used perfectly -- every day at the same time of day -- only 1 in 100 women have an unintended pregnancy during the first year of using the pill.

So yes, the pill works. But it's not ideal for women wanting a contraception method that they don't need to think about.

"It's a very effective method for women who remember to take their pill each and every day at around the same," says Elizabeth Micks, MD, an instructor and fellow in family planning at Oregon Health and Science University.

Birth control methods that don't require daily action on the woman's part, such as intrauterine devices (IUD) and contraceptive implants, have significantly lower failure rates.

If you've answered those questions and decided that you want birth control pills as your form of contraception, here are the options.

Types of Pills

There are two main types of birth control pills: combination pills and progestin-only pills. Most pills are available in both a 21-day or a 28-day pack. They are often classified on the amount of estrogen and/or the amount and type of progestin.

Combination pills. These contain the hormones estrogen and progestin. Most birth control pills are combination pills. They're equally effective at preventing pregnancy -- again, when taken correctly. They may also have other benefits, including:

• Shorter, lighter, more regular, and less painful periods
• May reduce the severity or frequency of menstrual migraines
• Can improve bleeding and pain associated with endometriosis and fibroids
• May improve acne
• Can lower risk of ovarian and uterine cancer
• Can lower risk of pelvic inflammatory disease
• May improve bone density in the years just before menopause

All combination pills slightly raise the risk of heart attacks, stroke, and blood clots that start in a leg vein but could travel to other parts of the body -- including the lungs, which could be fatal. That risk rises if you're a smoker older than 35.

Women who smoke and are older than 35 should not take any type of combination birth control pill, according to the FDA. Other factors -- such as being obese or having a family history of heart disease -- also make those clots more likely. So consider your overall risk.

Still, those clots are rare, overall.

"Somewhere on the order of two to four more women per 10,000 might experience a nonfatal [blood clot]," says Sarah Prager, MD, MAS, an assistant professor of obstetrics and gynecology at University of Washington Medical Center.

The FDA advises women not to take any combination birth control pills if they have a history of blood clots, heart attack, or stroke.

The FDA is also investigating whether certain combination pills that contain drospirenone (a synthetic version of progestin) carry a higher risk than other pills of deep vein thrombosis (DVT) or pulmonary embolism -- two rare but serious blood clots. Pills containing drospirenone include Beyaz, Gianvi, Loryna, Ocella, Safyral, Syeda, Yasmin, Yaz, and Zarah.

Progestin-only pills (also called the "mini pill"). These pills are most commonly used by nursing mothers, women with pre-existing risks for blood clots, or other conditions that prevent them from taking estrogen.

If you don't take them at the same time every day, they may not work. Being as little as three hours late could result in ovulation.

This type of pill works for nursing mothers because continuous breastfeeding already protects against pregnancy, and the mini pill simply provides added security. "Nursing moms have lower fertility. The mini pill's efficacy may be unacceptably low in women who have normal fertility," says Andrew Kaunitz, MD, who is associate chair of the ob-gyn department at the University of Florida College of Medicine in Jacksonville, Fla.

What's in That Pill Packet?

Combination birth control pills come in different phases, depending on whether the level of hormones in the pills changes throughout the month.

• Monophasic (one-phase) pills contain the same amount of estrogen and progestin in all of the active pills. Alesse, Loestrin, Ortho-cyclen, Seasonale, and Yaz are a few examples. Each active pill in the pack is the same. If you forget to take a pill one day, you take it as soon as you remember, and then take your next pill at your regularly scheduled time.
• Biphasic (two-phase) pills change the level of hormones estrogen and progestin once during the menstrual cycle. Examples include Kariva and Mircette Ortho-Novum 10/11.
• Triphasic (three-phase) pills contain three different doses of hormones in the active pills. Those levels change every seven days during the first three weeks of pills. This was the first type of birth control pill. Examples include Cyclessa, Ortho Tri-Cyclen, Nortel 7/7/7, Enpresse, and Ortho-Novum 7/7/7.
• Quadraphasic (four-phase) pills. The hormone levels in these pills change four times per cycle. Natazia is the only quadraphasic pill on the U.S. market.

Multi-phase pills (biphasic, triphasic, or quadraphasic) have two disadvantages, compared to monophasic pills:

1. They can be trickier to take. The more phases in the cycle, the more complicated the instructions are about what to do if you miss a day. Those instructions vary by pill.
2. Be careful not to start the next pack late. Except for Natazia, Kariva, and Mircette, all multi-phase pill cycles include seven hormone-free days. Starting the next pack late means more than a week without hormones, making pregnancy more likely. One of the most common ways women become pregnant on the pill is to start the next pack late.

Extended-Cycle, Continuous-Cycle Pills

Extended-cycle pills and continuous-cycle pills mean fewer periods, or even no periods. They include Introvate, Jolessa, Lybrel, LoSeasonique, Seasonale, and Seasonique.

Women may wonder if it's safe and healthy to skip periods. When a woman isn't taking oral contraceptives, a menstrual period is necessary after ovulation to shed the lining that's built up in her uterus.

Any birth control pill, regardless of cycle length, stops ovulation, so the lining of the uterus does not thicken -- and there is nothing to shed. So though you do have several planned days of bleeding on most pills, it's not really a menstrual period.

"Women often refer to the bleeding as menstruation, but in the case of birth control pills, it's withdrawal bleeding [from the hormone-free interval]," Kaunitz says. "So it's OK for women using hormonal birth control to bleed less often."

Longer cycles of pills simply cause this withdrawal less frequently, and many doctors encourage patients to try it.

"For a patient that wants to start birth control pills, I say, ‘And how about we miss periods, too?' I think that's appealing to more and more women," Micks tells WebMD.

But there are some drawbacks:

• You may be more likely to experience breakthrough bleeding than with other types of pills.
• It may be harder to tell if you get pregnant, because you're having fewer (or no) periods.
• Extended-cycle and continuous-cycle pills are not available in generic form. But doctors often instruct patients on how to adjust generic pills containing seven placebo pills for extended or continuous use. This will require a prescription for more than 12 packs per year.

Pill Safety

Each woman is different and may have different experiences with the various types of birth control pills. Some side effects may include:

• Light bleeding or "spotting" between periods. Spotting may be most common in pills with higher estrogen doses and cycles with higher numbers of active pills.
• Nausea
• Breast tenderness
• Weight gain

But unwanted side effects tend to go away in time, so gynecologists recommend patients stay with a pill for three to six months before quitting because of side effects.

Combination pills may not be right for you if you have:

• Risk factors for blood clots, stroke, or other cardiovascular conditions, such as women over 35 who smoke
• Had liver or breast cancer, although it has no impact on future risk for breast cancer
• Migraines with aura

Remember, although birth control pills make pelvic inflammatory disease less likely, they don't protect against sexually transmitted infections.

Once you start the pill, be careful with other prescription and over-the-counter drugs. The following medications can interfere:

• Antibiotics
• Certain seizure medications
• Certain HIV medications
• St. John's wort 

 Reviewed on March 19, 2012 © 2012 WebMD, LLC. All rights reserved.