“If a woman misses one pill every cycle, probably it’s not going to have a lot of impact on the pill’s effectiveness,” says Caroline Moreau, a Johns Hopkins epidemiologist specializing in fertility and contraceptive use. “Levels of hormones are going to be high enough that it’s going to suppress ovulation even if one pill is missed for 24 hours.”
An exception to that rule is progestin-only pills, which require taking the pill at a specific time of day.
But I had not missed a pill: My packs were clearly labeled by day, and it would have been easy to see if one had been skipped.
Was it metabolism?
So what other explanations might there be? Researchers are looking at how pills might perform differently depending on a person’s metabolism.
“We know that some people are fast metabolizers and some people are slow metabolizers,” says Alison Edelman, an associate professor of obstetrics and gynecology at the Oregon Health and Science University in Portland. A fast metabolizer might pass a drug through her liver too quickly; in the case of the pill, this could lower the hormone level below the critical threshold.
Was it obesity? Diet?
Another major question involves how obesity might affect the efficacy of the pill. Much of the research on the pill took place decades ago, prior to the obesity epidemic. In addition, clinical trials typically include the healthiest women and exclude those with health risks that might confound the studies.
“Obesity affects processes in the body that we cannot see or even fathom,” including how the body “sees” and responds to drugs, Edelman says. With the pill, obesity alters drug clearance, which could affect pill efficacy or increase the risk for failure when combined with a missed or late pill, she says.
Other potential culprits: Diarrhea or vomiting that causes foods and liquids to pass through the body rapidly, and changes in eating habits, such as adopting a grapefruit diet, which might alter absorption of the pill and affect hormone levels.
Contrary to what doctors often tell patients, common antibiotics such as amoxicillin and erythromycin have not been shown to interfere with the pill, Edelman says. Only “big gun” antibiotics such as rifampin — used to treat tuberculosis — are known to affect the efficacy of the pill.
I never did figure out why I got pregnant while on the pill. And that, researchers say, may just be how it is.
I’ll never know. I found peace with it and switched to an IUD. For good measure, my husband got a vasectomy, too.