It’s Science

11.26.13 6:00 PM ET

Why Plan B Might Not Work for Women Over 176 Pounds

The maker of a European morning after pill nearly identical to Plan B says its product may be ineffective for overweight women. The science behind the disappointing news.

Women and advocates for reproductive rights were greeted this week with dispiriting news: the so-called morning after pill—to be taken up to three or four days after unprotected sex—has been found to be ineffective in women who weigh more than 176 pounds and not dependably effective in those over 165 pounds.

In Europe, this will lead to a change in the product label for the Norlevo brand to warn women of the limitations of this birth-control method. In the United States, the FDA has not yet decided what course to take for Plan-B One Step, the only morning after product available in the U.S. without prescription. Plan-B One Step contains the same active ingredient in the same concentration as Norlevo. 

This means that the morning after option, which many women consider the least intrusive of birth control approaches, is not reliable for a substantial proportion of the population. According to Mother Jones, which broke the story, the average weight of American women older than 20 years of age is 166 pounds, and higher among black women. In all ways, this is an unhappy new development, pushing many women back to old choices like condoms, the pill, and the IUD. 

The science behind the unhappy development is as follows.

The morning after pill is not to be confused with the RU-486 (mifepristone) and mifostostol, which are abortifacients—used in early actual pregnancy to abort the fetus. Instead, morning after pills act before implantation of any fertilized egg. Both methods, however, use the same approach, indeed the same approach also used by standard birth control pills. The menstrual cycle and pregnancy are regulated and sustained by a delicate yin and yang of the female hormones estrogen and progesterone. Manipulating the dynamic balance between the two can either promote fertility or reduce it by preventing fertilization (the Pill) or implantation (morning after) of a fertilized egg, as well as provocation of the abortion (abortifacient) of an already implanted fetus. The specific effect depends on the dose and proportions of hormones used. 

The problem is that these hormones—the female sex hormones—are steroids. Not baseball-player PED steroids (those are sex hormones, too, but are androgens to enhance male-ness) or corticosteroids, such as prednisone (those are given to asthmatics to beat back an attack, and are different molecules altogether). Steroids—all of them, including female and male sex hormones—are concentrated and metabolized in fat cells. Not only that, but fat cells may themselves produce estrogen and other hormones.

In other words, fat cells are not sluggish lumps hanging out here and there but rather very active in the moment-to-moment regulatory work of the body, particularly the regulation of sex hormone levels.  

It is no surprise that the presence of more active fat cells, as is seen in someone who is overweight, would have a direct impact on blood and intracellular levels of the hormones given to prevent pregnancy. The likeliest explanation is that the fat cells in such persons have enhanced metabolism that leads to hormone levels that are too low in the overweight for the medication to be effective; alternatively, it may be that the essential estrogen-progesterone balance is disrupted, blunting the effect. 

Whatever the reason, this is yet another piece of bad health news for persons who are overweight. That said, in the reality-based world, facts are facts and must be respected until a new fact comes along.  So until a better understanding of why the morning after pill is less effective in woman above 165 pounds leads to an adjustment in the product, women in the U.S. above this weight are well advised to use an alternative approach to birth control.