eli5: Why do some antibiotics have the potential to make birth control not work?

Question

They seem to be unrelated at all.

In: 2

Answer 1

There’s not much concrete evidence they do affect it, but who’s going to volunteer to take the chance for a study where one of the outcomes is accidental pregnancy lol

Answer 2

Some forms of birth control are what’s called a “prodrug”. That means the chemicals in the pill you take are not the *actual* medicine. Rather, your body absorbs those chemicals, takes them to the liver, and they’re altered to then become the actual medicine. This is usually because the *active* medicine is likely to be poorly absorbed by the intestines and may be prone to being broken down by the acid in the stomach, but by making a few small tweaks we can get the medicine to sneak around those defenses.

Your liver has a bunch of machines in it that help convert things into other things (like prodrug -> drug and, importantly, estrogen -> broken down estrogen metabolite). That’s how we detoxify things. Some medicines have the potential to gum up the works and slow these machines down, others have the potential to ramp them up and make them work faster. Some antibiotics fall into that second category.

The result is a liver which is revved up and breaks down estrogen much faster, and because the birth control is all about controlling hormone levels, the birth control becomes less effective. This doesn’t apply to all antibiotics, just some of them.

Answer 3

With the amount of drugs available it’s very common for people to be on two or more drugs at the same time. So, it is important to understand the possible reactions between drugs.

One of the functions of the liver is to metabolize drugs. One of the systems it uses for that is called the CYP450 system. Oral birth control pills are metabolized by the CYP450 system. So if any drugs make that system stronger then more birth control will be metabolized, making it less effective.

Some antibiotics do exactly that. They increase the effect of CYP450, which in turn reduces the effect of birth control pills.

There are many drugs that induce and inhibit that system so the potential drug-drug reactions are very large.