What are side effects for missing a pill of birth control?

The effects of missing a birth control pill depend on when in the package the pill is missed, what the dose of estrogen is in the pill and what type of pill you are taking.

The most important pill in the package as far as effectiveness is the first pill. Pills are organized with a number of active or hormone containing pills, followed by placebo or hormone free pills. Some brands have 21 days of hormones and 7 days of placebo and others have 24 days of hormones followed by 4 days of placebo. Your body already begins to develop a follicle or egg for the next cycle during the placebo pills. In some women the egg is already nearly mature when she starts her new pack of pills and stops its development. Starting a new pack of pills even a day late will allow some women to progress to ovulation and thus a chance of unintended pregnancy.

Missing a pill after the first week is not likely to allow a pregnancy. The break in hormone levels, however, may destabilize the lining of the uterus and allow vaginal spotting or even light bleeding. This spotting may continue through the end of that cycle. Pills with very low doses of estrogen, such as 20 micrograms (mcg), are more susceptible to a missed pill causing spotting than pills with slightly higher doses such as 30 or 35 mcg. 

Finally, pills that contain only a progestin, sometimes called the "mini-pill" depend on a very steady dosing to prevent pregnancy. The package insert recommends that if a pill dose is late by more than three hours, a back up method of contraception such as condoms should be used for seven days.

Women who have missed a pill should use a back up contraception for seven days. Women who have missed two pills in a row should use emergency contraception and a back up contraception for seven days. Women who regularly miss pills might consider using a different birth control method such as a vaginal ring, a patch, or an IUD. These methods do not require daily attention.