Medically reviewed by Dr. Nancy Shannon, MD, PhD on January 13, 2020
The length of time it will take for your birth control pills to prevent pregnancy can range from immediately to seven days, depending on the type of oral contraceptive. After the initial waiting period, birth control pills can be 99 percent effective at preventing pregnancy when taken exactly as directed and 91 percent effective with typical use.
How Long Does it Take a Progestin-only Pill to Work?
When prescribed a progestin-only pill, you will likely be instructed to take it within the first five days of the start of your cycle, which is typically on the first day of your period.
Unless you have a shorter cycle, 23 days or less, the pill will become effective immediately if you begin taking it within the correct time frame. If you do have a shortened cycle, it can take two full days for the pill to become effective at preventing pregnancy.
How Long Does it Take a Combination Pill to Work?
If you start the combination pill on the first day of your period, you should be protected right away. If you start the combination pill at any other time during or after your period, it can take a week to become fully effective, so use a backup form of contraceptive during this time to avoid getting pregnant.
If you’re already on a combination pill and just switching brands, you’ll still be protected as long as you don’t skip a day between pills in the process.
How Does the Pill Work?
There are two types of birth control pills: the progestin-only (or mini-pill) and the combination pill. Each works in similar ways using different types and amounts of hormones. You must take either type every day to prevent pregnancy.
Combination pills contain both the hormones estrogen and progestin, which work together to prevent ovulation from occurring. They also cause the cervical mucus to thicken and the uterine lining to thin to further prevent sperm from reaching an egg. They’re upward of 91 percent effective.
A month’s worth of combination pills includes 21 active pills (meaning they contain hormones) and one week of inactive (no hormones) pills. Some types of combination pills have 24 active and four inactive pills. Women typically get their period during the week of inactive pills. They’re still protected against getting pregnant, however, during this time.
For the best protection, take your combination pill at the same time every day. As long as you take them within the same 12-hour period each day, however, you should be protected against getting pregnant.
Progestin-only pills work primarily by thickening the cervical mucus and thinning the uterine lining. Less commonly, they also reduce a woman’s chances of ovulating. They’re at least 95 percent effective when taken properly.
Mini-pills come in sets of 28 active pills. It’s particularly important to take these pills at precisely the same time every day. If you miss this window by three or more hours, you might begin ovulating. If you’re late to take the mini-pill, use a backup form of birth control, such as condoms, for the next two days to avoid getting pregnant.
Take any type of birth control pill according to the label and your healthcare provider’s instructions, even when you’re not having sex.
Many women report experiencing some mild side effects after starting the pill, most of which go away with time. These can include:
- Changes in the duration or flow of your period
- Missed periods
- Spotting between periods
- Vaginal discharge
- Mood changes
- Headaches or migraines
- Breast tenderness
- Reduced libido
Women with a history of stroke or high blood pressure or those who smoke might be at risk of more serious side effects such as blood clots, stroke, high blood pressure, and heart attacks. To make sure you’re not at risk, your healthcare provider will want to review your medical history before prescribing birth control.
Factors That Can Affect the Pill’s Efficacy
Once you’re on the pill, note that some medications can make it less effective. These include:
- Rifampin, an antibiotic used to treat tuberculosis
- Antiseizure medications such as carbamazepine and topiramate
- Antiretroviral medications, such as lopinavir and saquinavir, prescribed to manage HIV
- Herbal supplements, such as St. John’s wort
Vomiting and diarrhea can also affect your pill’s effectiveness. Let your healthcare provider know if you’re taking any other medications in conjunction with your birth control.
Other Types of Birth Control
Other forms of birth control begin working sooner or later than the pill. These include:
- Hormonal intrauterine device (IUD) – IUDs that contain hormones start working within seven days of insertion.
- Copper IUD – These devices are effective at preventing pregnancy immediately after being inserted.
- The implant – Birth control implants are effective immediately if inserted within five days of starting your period. Otherwise, they take seven days to start working.
- The shot – Similar to implants, a shot such as Depo-subQ Provera 104 is effective within 24 hours if injected within five days of starting your period. Otherwise, it takes seven days to reach full efficacy.
- The patch – If you’re wearing a patch such as Xulane for the first time, it takes seven days for it to be fully effective.
- The ring – Vaginal rings such as NuvaRing are effective immediately if you insert them on the first day of your period. Otherwise, it takes them seven days to become effective.
- Sterilization – Also known as tubal ligation, permanent female sterilization is effective immediately and irreversible.
- Barrier methods – Condoms, diaphragms, cervical caps, and sponges are also effective immediately if you’re wearing them properly and they fit well.
To prevent pregnancy, always use a backup form of birth control, such as condoms, diaphragms, sponges, or cervical caps, during the transition period after starting your birth control.