One of the factors that can change the effectiveness of birth control can be timing. Birth control pills are best taken on a regular schedule, as if they were prescribed medication. Sticking to a certain “rhythm” with your birth control pills can help in maintaining their ability to prevent pregnancy, particularly if the pill only works on progestin. Most people don't realize this and will take the pill during different times of the day, which can result in a decrease in how effective they are. Maintaining a regularly scheduled intake of one's medication should work to maximize the effectiveness of birth control pills. It should be noted that they are not an absolute guarantee that pregnancy will not happen, but it will minimize the chances of such an occurrence.
Some antibiotics can also have an effect on how effective one's birth control medication can be. Rifampin, an antibiotic medication usually used to combat conditions like tuberculosis, is known to interfere with the effectiveness of birth control pills. Most other antibiotics should be fine, though demeclocyclene and doxycyclene should be mentioned. While not as rampant in side effects as Rifampin, the two medications can also have conflicts with birth control medication. In this particular case, it is best you consult your doctor about what antibiotics would not interfere with your current contraception plan.
Grapefruits, oddly enough, can also have an effect on how effective a contraceptive is. There is a compound within the grapefruits that absorbs estrogen. Since most birth control medications rely on manipulating or altering the estrogen levels of the body, along with a few other changes to hormone levels, this can disrupt the effectiveness of your pills. It should be noted that grapefruit juice also has the same compound and has an effect that is roughly on-par with eating the grapefruit itself. It should also be noted that progestin-only pills are not hampered by this, though as stated above, timing could be an issue for them.
Antifungal medications, antihistamines, and anticonvulsant drugs can also have effects on how effective a birth control pill would be in the body. In the case of antihistamines, there is still a debate going on, because there is no direct evidence pointing one way or another. Antifungal medications that are introduced to the body orally, rather than applied topically, can disrupt some of the biochemical changes that contraceptive pills make. Finally, not all anticonvulsants can lessen the chances of a birth control pill working, so it is best to consult a medical professional to see which ones might have such an effect.
