Birth control pills contain synthetic hormones that mimic the ones your body naturally produces, specifically estrogen and progesterone. Depending on the type of pill, you’ll find either one or both of these hormones as the active ingredients, along with a handful of inactive fillers that hold the pill together and help your body absorb it.
The Two Types of Active Hormones
There are two main categories of birth control pills, and they differ by what’s inside. Combination pills contain both a synthetic estrogen and a synthetic form of progesterone called a progestin. The mini-pill contains only a progestin with no estrogen at all.
The most common synthetic estrogen used in combination pills is ethinyl estradiol, which has been the standard for decades. A newer option called estetrol, modeled after an estrogen naturally present during pregnancy, was approved by the FDA in 2021 as part of a combination pill. It was developed partly because it may interact differently with the liver compared to ethinyl estradiol.
On the progestin side, there’s more variety. Three progestin-only formulations are currently available in the United States: norethindrone, norgestrel, and drospirenone. Combination pills use these and other progestins as well. Different progestins can have slightly different effects on things like acne, bloating, and mood, which is one reason there are so many pill brands on the market despite all of them working in fundamentally similar ways.
How These Hormones Prevent Pregnancy
Combination pills work primarily by stopping your ovaries from releasing an egg each month. No egg means nothing for sperm to fertilize. The progestin component also thickens the mucus at the opening of the cervix, creating a barrier that makes it much harder for sperm to reach an egg in the first place.
The mini-pill relies more heavily on that cervical mucus effect, though it can also disrupt ovulation in some cycles. Because it only contains one hormone at a lower dose, the mini-pill is often recommended for people who can’t take estrogen, such as those with a history of blood clots or who are breastfeeding.
How Hormone Doses Change Through the Pack
Not all pills in a pack deliver the same amount of hormone. In a monophasic pill, every active pill contains the same dose of estrogen and progestin. This is the simplest design and makes it straightforward if you accidentally take pills out of order.
Biphasic and triphasic pills change the hormone levels partway through the pack. In a biphasic pill, the dose shifts once. In a triphasic pill, it shifts twice, creating three distinct phases. The idea behind these designs is to more closely mirror the natural rise and fall of hormones during a menstrual cycle. In practice, the pills in these packs are usually color-coded so you take them in the correct sequence.
Inactive Ingredients in Every Pill
The hormones are only a tiny fraction of what’s physically in each tablet. The rest is made up of inactive ingredients, sometimes called excipients, that serve practical purposes: helping the pill hold its shape, dissolve properly in your stomach, taste acceptable, and last on a shelf without breaking down.
Common inactive ingredients include lactose (a milk sugar used as a filler), corn starch (which helps the pill disintegrate once swallowed), and magnesium stearate (a lubricant that keeps the pill from sticking to manufacturing equipment). Many pills also contain food dyes, which is why different brands come in different colors. Roughly 45 percent of medications contain lactose, and about 33 percent contain a food dye.
These ingredients matter more than most people realize. Researchers have identified at least 38 inactive ingredients across medications that can trigger allergic reactions in sensitive individuals. Lactose intolerance is the most commonly relevant one for birth control users, though the amount of lactose in a single pill is very small. A less common but more serious concern is peanut oil, which appears in a small number of progesterone formulations. If you have a known food allergy, checking the full ingredient list on the package insert is worthwhile.
What’s in the Placebo Pills
Most combination pill packs come with 21 active pills and 7 inactive ones, though some newer packs use 24 active and 4 inactive, or skip the inactive pills entirely for continuous use. The inactive pills are often called “sugar pills,” though they’re not actually made of sugar. They contain no hormones and exist purely as reminders to keep you in the habit of taking a pill every day.
Some brands add a small iron supplement to the placebo pills. This can help offset the iron you lose during the withdrawal bleed that typically happens during the placebo week. The iron isn’t doing anything for pregnancy prevention. It’s just a nutritional bonus built into pills you’d be taking anyway. If your pack’s placebo pills are a different color from the active ones, that’s your visual cue for which phase of the pack you’re in.
Why There Are So Many Brands
With only two basic hormone types (estrogen and progestin), you might wonder why there are dozens of birth control pill brands available. The answer comes down to combinations. Different progestins paired with different estrogen doses at different phasic schedules create distinct formulations, each with a slightly different side effect profile. One combination might work well for someone prone to hormonal acne, while another might cause fewer headaches or less breakthrough bleeding.
The inactive ingredients also vary between brands and between brand-name and generic versions. Two pills with identical hormones can differ in their fillers, dyes, and coatings. For most people this makes no practical difference, but for those with specific allergies or sensitivities, it can be the reason one brand feels fine and another doesn’t.