Birth control pills are an effective way to prevent pregnancy. However, missing pill days, vomiting, and certain medications, among other factors, can reduce its effectiveness. This could lead to an unintended pregnancy.

Anyone who is worried that they might not be able to take the pill correctly should discuss other birth control options with a healthcare professional.

In this article, we look at how effective the birth control pill is and give five reasons why the pill might fail. We also provide tips on how to prevent pill failure and describe some early signs of pregnancy.

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Birth control refers to the use of various devices, drugs, agents, sexual practices, or surgical procedures to prevent conception or pregnancy. One such method is the combined pill, also known simply as the pill.

The combined pill contains hormones that prevent ovulation, which is when the ovaries release an egg for fertilization. Another type of pill, known as the minipill, causes a person’s cervical mucus to thicken and the uterine lining to thin, which reduces the likelihood of sperm reaching an egg.

The birth control pill is very effective if a person takes it correctly and does not miss any pill days.

According to the National Health System (NHS) in the UK, the pill is 99% effective with perfect use. This means that fewer than 1 in 100 people who take the pill may become pregnant in 1 year. However, with typical use, the effectiveness of the pill is 91%, meaning that about 9 in 100 people may become pregnant in a year of taking it.

How does the pill compare with other birth control methods?

There are different types of birth control methods, some of which are hormone-based and some of which are not. The effectiveness of each type also varies according to the limitations of typical use. Certain types of birth control may be a better fit for a person than others.

In addition to the combination birth control pill, other contraceptive methods include:

In addition, there are a few male contraceptive options, but these are much more limited. They include condoms, vasectomy, and the withdrawal method, which is not particularly reliable. Researchers are investigating a male birth control pill, which is in clinical trials.

If a person is concerned about contraceptive failure and the possibility of becoming pregnant, they should consult a doctor. It may also be helpful to be aware of the early signs of pregnancy, which can include:

  • Upset stomach: Some people may experience vomiting, known as morning sickness, alongside this.
  • Breast tenderness: Raised levels of the hormones estrogen and progesterone can cause breast tenderness early in pregnancy. Some people may also experience symptoms, such as tingling, heaviness, or feelings of breast fullness.
  • Light spotting: A small amount of bleeding or spotting can occur when an egg attaches to the uterine lining. If spotting occurs outside of an expected menstrual cycle, it may be an early symptom of pregnancy.
  • Unexplained fatigue: Hormonal changes that occur in the body during pregnancy can make a person feel tired, even during the early stages.
  • Urinating more often: Hormonal changes can increase the need to urinate during early pregnancy.

Anyone who thinks that they could be pregnant can consider taking an at-home pregnancy test. In recent years, these tests have become more sensitive to a person’s hormone levels and better at detecting pregnancy in its earliest stages.

However, at-home pregnancy tests still may not be reliable if people use them too early or do not follow the instructions correctly. According to the Food and Drug Administration (FDA), for the most reliable results, a person should take the test 1–2 weeks after their first missed period.

Although the birth control pill generally works very well, some situations can reduce its effectiveness, and they may sometimes result in an unintended pregnancy. These include:

Missing a day

Manufacturers intend for people to take the pill daily. If a person misses a day, their hormone levels may not remain at consistent enough levels to prevent pregnancy.

If a person finds it difficult to take the pill on a daily basis, other birth control methods may better suit their needs. A doctor or gynecologist can advise on the range of alternative contraceptives.

Vomiting or severe diarrhea

Sometimes a person may be ill when they take the pill. When a person vomits, the pill can come back up, or they may not fully absorb it into their body. The latter may also occur if a person has severe diarrhea.

Anyone who experiences vomiting or diarrhea shortly after taking the pill should take another pill as soon as possible and then take their next pill as usual.

Not taking the pills at the same time each day

In addition to taking birth control pills daily, a person should take the pills at about the same time each day. Doing so can maintain their hormone levels more consistently.

A person should always take the minipill within the same 3-hour time window every day. Someone who misses their usual window should use a backup birth control method for the next 7 days or avoid having sex.

Many people set a daily alarm reminding them to take their pill at the correct time each day.

Not starting a new pack right away

It is essential to start a new pack of pills the day after finishing the previous one. However, a person may not always have their new pack in time. Missing a few days between packs can make the pill less effective in preventing pregnancy.

According to the CDC, anyone who misses two or more pills in a row should use a backup birth control method or avoid sexual intercourse until they have taken the birth control pill for 7 consecutive days.

Medications that interfere with the pill

Some medications can make the pill less effective. These medications include certain antibiotics, such as rifampicin, and antifungal drugs, such as griseofulvin.

A person should use backup contraception while taking these medications and for 48 hours after finishing the course.

Other long-term medications and supplements may also affect how well birth control pills work. These include:

  • epilepsy drugs, such as phenobarbital, phenytoin, and carbamazepine
  • antiviral medications for the treatment of HIV
  • St. John’s Wort, which is a herbal remedy

Birth control pills are very effective if a person takes them correctly and does not miss any pill days. People can also take steps to prevent unintended pregnancies while taking the pill. These include:

  • reading the packaging and following the instructions carefully
  • taking the pill at the same time every day
  • using an app that tracks periods and provides pills reminders, such as these period tracking apps
  • always getting a new pill pack at least 1 week before the last pill pack is due to run out
  • always taking missed pills as soon as possible
  • using a backup method of contraception, such as a condom, if a person misses two or more pills in a row

If a person is concerned about not being able to take their pills consistently, they should speak with a doctor or gynecologist about other birth control methods. Several options are available that do not require people to take a pill daily, such as an IUD.

Below, we answer some of the questions that people often ask about birth control pills.

What is the possibility of getting pregnant while on birth control?

The chance of birth control being ineffective depends on the type of birth control and whether a person is using the method correctly. However, the only birth control method that is 100% effective is abstinence.

How did I get pregnant on the pill?

Even when taking the pill, if a person has sexual intercourse, there is still a chance of pregnancy. For instance, with the typical use of the combination birth control pill, there is still a 9% chance of pregnancy.

What should you do if you become pregnant while taking the pill?

If a person becomes pregnant, they should stop taking the birth control pill. However, research shows that birth control pills pose little threat to a developing fetus. Learn how to recognize pregnancy symptoms while taking the pill.

Can you get pregnant on the pill despite using the withdrawal method?

As long as sexual interaction takes place and sperm enters the vagina, there is a chance of pregnancy even while taking the pill. The withdrawal method is also not 100% effective, which means that pulling out while on the pill does not reduce the chance of pregnancy to 0%.

Can you get pregnant if you miss one pill?

Missing a pill means that more than 24 hours have passed since the last time a person has taken a dose. The likelihood of pregnancy rises if a person skips a pill. A person should take the missed pill as soon as they remember and use another birth control method, such as condoms, for 7 days.

How effective are birth control and condoms together?

Even with typical use, rather than ideal use, many hormonal and device-based contraceptive methods are highly effective on their own, so combining them with a condom is likely to reduce the chance of pregnancy further. However, the main reason to use condoms alongside a person’s birth control method of choice is that other methods do not protect against sexually transmitted infections (STIs).

Although birth control pills are generally very effective, they can sometimes fail to prevent pregnancy if a person does not use them correctly and consistently.

Anyone who is concerned about the effectiveness or convenience of their method of birth control should speak with a doctor.

If a person misses more than one pill, they should use a backup contraceptive method alongside the pill for at least 7 consecutive days.