How much less effective is a low-dosage birth control pill compared to a higher-dosage pill? Is the weaker one less effective than the stronger? What considerations go into deciding the strength of the dosage a patient should take?

--Sharlene

All pills available today are "low dose," especially when compared with the original pills first available in the 1960s. Those pills contained 150mcg of ethinyl estradiol (estrogen), whereas most pills today have 20-35 mcg, with a few containing 50 mcg. If taken correctly, a 20 mcg pill is as effective as one with more estrogen. All the hormones in the world will not prevent pregnancy if the pill is not taken properly.

The original birth control pills that were tested in the late 1950s and early 1960s actually did not have any estrogen at all—just progestin (synthetic progesterone), the other hormone in today's pills. Estrogen was added not to increase effectiveness, but rather to decrease an annoying side effect of any progestin-only method (including Depo-Provera)—irregular bleeding. Adding estrogen lessens breakthrough bleeding and creates extremely regular cycles.

All combination oral contraceptive pills on the market in the United States contain the same estrogen— ethinyl estradiol. The progestin component of the pill varies among manufacturers. The type of progestin and the dose of estrogen play a part in side effects; the differences, however, are basically minor and subtle, despite the drug companies' advertisements. Some pills are monophasic, containing the same dose of estrogen and progestin in all 21 active pills, while others are multiphasic, with the dose of one or both components varying throughout the cycle.

As for which pill is best for which person, that is a matter of trial and error. If a woman has never been on the pill before, it really doesn't matter much which of today's pills she is placed on. In general, the 50 mcg estrogen pills are not prescribed right off the bat, but are reserved for women who have breakthrough bleeding on the lower estrogen pills. If a women has side effects on one prescription, she can try another type; for instance, it is usually the estrogen component that contributes to nausea, so if a woman becomes queasy on a 35 mcg pill, then her doctor may switch her to a 20 mcg pill. In general, a particular pill is tried for three cycles before a change is made. This allows the woman to get used to the pill, and most minor side effects will disappear in this time frame. Good communication between doctor and patient is key in finding the best pill—or any other contraceptive option—for a particular individual.