Plan B is a single high-dose hormone pill taken after unprotected sex to prevent pregnancy, while daily birth control pills are low-dose hormones taken every day to provide ongoing protection. They use similar hormones but work on completely different timelines, have different effectiveness rates, and serve different purposes.
How Each One Works
Daily birth control pills prevent pregnancy through three overlapping mechanisms. They stop your ovaries from releasing an egg, thicken the mucus at the cervix so sperm can’t pass through easily, and thin the uterine lining. Combined pills contain two hormones (estrogen and progestin), while the mini-pill uses only progestin. Either way, you take one pill at the same time every day, creating a steady hormonal environment that keeps ovulation suppressed.
Plan B works differently because it’s solving a different problem. It delivers a single large dose of levonorgestrel (the same progestin found in many daily pills, just at a much higher concentration) to delay or block ovulation after sex has already happened. If your body hasn’t released an egg yet, Plan B can hit the brakes on that process so the egg and sperm never meet. It does not end an existing pregnancy.
Effectiveness: Ongoing vs. One-Time
Daily birth control pills have a typical-use failure rate of about 7% per year, according to the CDC. That means out of 100 people relying on the pill for a full year, roughly 7 will become pregnant, mostly due to missed pills or inconsistent timing. With perfect use, the failure rate drops well below that.
Plan B is 81 to 90% effective at preventing pregnancy from a single act of unprotected sex, depending on how quickly you take it. It works best within the first 24 hours and remains effective up to 72 hours (three days). You can still take it between 73 and 120 hours after sex, but effectiveness drops noticeably in that window. This makes timing critical in a way that daily pills don’t require, since those build consistent protection over time.
Body Weight Affects Plan B More
One important difference that many people don’t realize: Plan B becomes significantly less effective at higher body weights. Research shows that levonorgestrel-based emergency contraception starts losing effectiveness at around 70 kg (about 154 pounds) and may have essentially no efficacy at 80 kg (176 pounds) or above. People with a BMI of 30 or higher who take Plan B achieve peak hormone levels roughly 50% lower than those with a BMI under 25, which likely explains the higher failure rates.
Daily birth control pills don’t have this same weight-related drop in effectiveness. If you weigh more than 155 pounds and need emergency contraception, a copper IUD (which can be placed up to five days after unprotected sex) or a different emergency contraceptive pill may be more reliable options worth discussing with a provider.
Side Effects
Both methods can cause side effects, but the experience feels quite different. Daily birth control pills introduce hormones gradually, so side effects like mild nausea, breast tenderness, or spotting tend to appear in the first few months and then settle down as your body adjusts.
Plan B delivers a concentrated hormone surge all at once, so side effects tend to be more noticeable but short-lived. Common ones include nausea, fatigue, headache, dizziness, breast tenderness, abdominal cramping, and spotting. Your next period may come earlier or later than expected, and the cycle after that is usually back to normal. Most people find these side effects mild.
Can You Use Plan B Regularly?
Repeated use of Plan B poses no known serious health risks, according to the World Health Organization. However, frequent use can cause ongoing menstrual irregularities, and it’s simply less effective than daily contraception. A method that fails 10 to 19% of the time per use will add up to a much higher chance of pregnancy over months compared to a daily pill with a 7% annual failure rate. Plan B also costs more per use than a month of birth control pills, making it an expensive long-term strategy.
Cost and Access
Plan B has been available over the counter without a prescription or age restriction since 2013. You can buy it at most pharmacies, and multiple generic versions exist. A single dose typically costs $20 to $50 out of pocket.
Daily birth control pills require a prescription. Under the Affordable Care Act, most marketplace health insurance plans cover FDA-approved contraceptive methods, including both birth control pills and emergency contraception like Plan B, with no copay when prescribed and obtained through an in-network provider. If you don’t have insurance or your employer has a religious exemption, you’d pay out of pocket for either method.
Which One You Actually Need
These two options fill completely different roles. Daily birth control is your baseline protection: consistent, reliable, and designed for ongoing use. Plan B is a backup for situations where your regular method failed or wasn’t used. A condom broke, you missed multiple pills, or sex was unprotected for another reason.
Taking Plan B does not interfere with your regular birth control. If you’re already on the pill and had a lapse (like missing two or more days), you can take Plan B and then resume your daily pills as normal. Plan B does not replace your regular method going forward, and it won’t protect you from future unprotected sex in the same cycle.