Several things can reduce how well birth control works, from missed pills and medication interactions to vomiting, certain supplements, and even how you store your pills. The gap between “perfect use” and “typical use” is significant: oral contraceptives have a failure rate of about 5.5 per 100 users in real-world conditions, compared to less than 1 per 100 with perfect use. Understanding what interferes with your contraception helps you stay on the right side of that gap.
Missed or Mistimed Pills
The single most common reason birth control pills fail is inconsistent use. Combined pills (estrogen plus progestin) and progestin-only pills both depend on maintaining steady hormone levels in your body. Missing even one pill, or taking it several hours late, can allow your body to begin preparing for ovulation. Progestin-only pills have an especially narrow window: many need to be taken within the same three-hour block each day to remain reliable.
This is the main driver behind the difference between perfect-use and typical-use failure rates. Perfect use means taking the pill at roughly the same time every single day, never missing a dose. Typical use accounts for the reality that people forget, travel across time zones, or occasionally run out of refills. If consistency is difficult for you, longer-acting methods like an IUD, implant, or injection remove the daily variable entirely.
Medications That Speed Up Hormone Breakdown
Certain prescription drugs make your liver process contraceptive hormones faster than normal, lowering the amount circulating in your blood. The result is hormone levels too low to reliably prevent ovulation.
The biggest culprits are enzyme-inducing seizure medications. Phenytoin, carbamazepine, topiramate, phenobarbital, and oxcarbazepine all fall into this category. Women using hormonal birth control alongside these drugs have substantially higher rates of unintended pregnancy compared to women on seizure medications that don’t trigger this effect (like lamotrigine, gabapentin, or levetiracetam). If you take any seizure medication, it’s worth confirming with your prescriber whether it interacts with your contraception.
Some HIV medications and the antifungal griseofulvin work through the same liver pathway and can similarly reduce contraceptive hormone levels.
Antibiotics: Mostly Safe, With One Major Exception
The idea that antibiotics cancel out birth control is widespread, but the reality is more specific. Common antibiotics like amoxicillin, azithromycin, and doxycycline have not been shown to reduce contraceptive effectiveness.
The clear exception is rifampin, an antibiotic used primarily for tuberculosis. Rifampin dramatically accelerates the breakdown of both estrogen and progestin. In one study, half of the participants ovulated while taking their birth control pill alongside rifampin, compared to zero who ovulated on the pill alone. Across multiple studies, progestin levels dropped by 30 to 83 percent and estrogen levels dropped by 42 to 66 percent when combined with rifampin. Rifabutin, a related drug, causes smaller but still notable reductions in hormone levels. If you’re prescribed either of these, you’ll need a backup method or an alternative form of contraception.
Another related antibiotic, rifaximin (commonly used for traveler’s diarrhea), did not alter hormone levels in testing and does not appear to pose the same risk.
St. John’s Wort and Herbal Supplements
St. John’s Wort, a popular over-the-counter supplement used for mild depression and anxiety, is one of the most well-documented herbal threats to birth control. It activates the same liver enzymes that rifampin does, speeding up hormone metabolism. In clinical testing, women taking St. John’s Wort alongside oral contraceptives showed significantly faster clearance of progestin and a halved duration of estrogen activity (from about 23 hours down to 12 hours). That’s a meaningful drop that could allow breakthrough ovulation.
Because St. John’s Wort is sold as a supplement rather than a prescription drug, many people don’t think to mention it to their prescriber or pharmacist. If you rely on hormonal birth control, this is one supplement worth avoiding or discussing with a healthcare provider first.
Vomiting and Diarrhea
Birth control pills need to be absorbed through your digestive tract to work, and anything that disrupts that process can leave you unprotected. The NHS advises that if you vomit within three hours of taking a combined pill, your body may not have absorbed enough of the dose. You should take another pill right away.
Severe diarrhea lasting more than 24 hours also poses a risk to absorption. In that case, continue taking your pills on schedule but use a backup method like condoms until seven days after the diarrhea stops. This is one reason non-oral options (patches, rings, injections, implants, IUDs) can be a better fit for people who frequently deal with stomach illness.
Body Weight and Emergency Contraception
Body weight has a more limited effect on daily birth control pills than many people assume, but it plays a clearer role in emergency contraception. Levonorgestrel-based emergency contraception (Plan B and its generics) becomes less effective at higher body weights. A 2011 meta-analysis found that women classified as obese had four times the risk of pregnancy after taking it compared to women at a lower weight.
Health Canada issued guidance stating that levonorgestrel may be less effective in women over 165 pounds and possibly ineffective over 176 pounds. That said, other health authorities have been more cautious about drawing hard cutoffs, noting that some protection is still likely better than none when other options aren’t accessible. The copper IUD, when inserted within five days of unprotected sex, is the most effective form of emergency contraception regardless of weight. Ella (ulipristal acetate), another prescription emergency option, maintains better effectiveness at higher weights than levonorgestrel does.
Digestive Conditions and Absorption
Chronic conditions affecting the small intestine, such as Crohn’s disease or celiac disease, raise theoretical concerns about whether oral contraceptives are fully absorbed. In Crohn’s disease, intestinal inflammation, ulceration, or prior surgical removal of bowel segments could all limit how much hormone makes it into the bloodstream.
The actual data is reassuring, though. A study tracking contraceptive failure in women with Crohn’s disease found a pregnancy rate of 0.5 percent, which is close to the 0.3 percent rate seen with perfect use in the general population. None of the women who became pregnant in that study had a history of bowel resection or celiac disease. Still, if you have active small bowel disease or have had significant intestinal surgery, non-oral methods bypass the gut entirely and eliminate that variable.
Heat, Storage, and Expired Pills
Hormonal medications are formulated to remain stable at 25°C (77°F) or below. Exposure to higher temperatures can degrade the active ingredients and reduce potency. This matters if you leave pills in a hot car, store them in a steamy bathroom, or travel in extreme heat without keeping them in a temperature-controlled environment. Freezing can also affect stability, particularly for liquid or gel-based formulations. Storing your pills in a cool, dry spot at room temperature is the simplest way to protect them. Using pills past their expiration date carries similar risks, since the hormones may have degraded below therapeutic levels.
Timing and Method Mistakes
Beyond pills, other hormonal methods have their own user-dependent failure points. The contraceptive patch needs to be replaced on schedule, and it can partially detach without you noticing, especially during exercise or in humid conditions. The vaginal ring must be kept in place for three consecutive weeks; removing it for more than three hours during that window can reduce protection. Injectable contraception (the shot) needs to be repeated every 12 to 13 weeks, and falling behind schedule opens a gap in coverage.
For barrier methods like condoms, incorrect use is the primary failure mode. Using the wrong size, not leaving space at the tip, using oil-based lubricants with latex (which degrades the material), or putting a condom on after intercourse has already started all reduce effectiveness significantly.