Why does birth control efficacy go down over several years of use?

[Digdawg]

Hi Scarleteen!

This is purely a curiosity based question, although I figure this could be useful in reevaluating whether I want to switch to a more effective contraception.

So I know that efficacy rates are determined over a period of one year through clinical study and surveys. But it seems that none of these studies extend one year of observation. So I went googling and found this article by the New York Times: https://www.nytimes.com/interactive/201 ... ncies.html
I read it and it seems that over ten years, the failure rates for typical use of barrier, behavioral, and most short term hormonal methods (like the pill or the patch) seem to skyrocket. For example, I have been on the combined pill for four years now, and I’m really good about taking it every day. I know that I have not missed a pill more than once or twice, and that was over three years ago (and I was also not sexually active at the time). So all in all, I’d like to consider myself a perfect user, or at least very close. But because you can never be sure and humans are not perfect, I recognize that it may be more beneficial to refer to the typical use rates to get a confident minimum efficacy rate. When looking at this article, for the pill, perfect use failures after four years of use remain 1 in 100, but typical use rates jump to 31 in 100, from the 9 in 100 from one year of use. Why is this? Where do they get their numbers? Is everyone guaranteed a higher risk of pregnancy after so long using certain contraceptions certain ways? How does combining methods factor into all this? How would calculate the efficacy of multiple methods after multiple years of use?

Thanks for your input!

[Mo]

I don't have a subscription so can't see past the paywall, but it sounds like what the article is stating is that it isn't that birth control methods themselves become less effective, but that people become more lax with keeping up with those methods over time. "Typical use" covers all the way that people use their birth control methods in a year, including using them incorrectly or inconsistently. We discuss typical vs. perfect use in Have a Little Faith in BC, which says:

Effectiveness rates of contraceptive methods are all figured based on one full year of use, not for each single incident of sex or use of a method. Perfect, or proper, use of a method means that in one year, that method was always used, and always used following the directions for that method to the letter, without any errors or mistakes. Perfect use of most methods in one full year is unusual, except for methods like the IUD or implant where a user does not have to do anything. Typical use means that in one year, sometimes a method has not been used according to the directions, or was not always used. Typical use is called that because that is most typically how methods are used in a full year, since people aren't perfect, and most often, in a year, will tend to make a mistake now and then with a method.

So you can probably imagine that someone who's say, taking birth control pills might have a very strict regimen the first year or so, where they have an alarm and take them at the same time every day, because they're nervous about starting a new method, but after a few years their schedule changes or they feel more confident in their ability to remember so they no longer have an alarm, so they take the pill a few hours late sometimes...and maybe they miss one or two here or there. Not everyone will do this, of course! Plenty of folks are consistent with their usage over time. But a longer period of time means more opportunities for error.

None of this changes how effective these methods are if they're being used according to the directions! It has to do with human error. So, as long as you are using your method or methods as directed, you shouldn't see any reduction in efficacy, whether you're using multiple methods, using one for a year or ten, etc.

[Digdawg]

Good to hear!

I kind of figured that the numbers shouldn’t change much, if at all, with long term use when used correctly.

I also found in the footnotes of the article that they got these numbers by raising a certain rate to a power equal to the number of years of use. So if you have been on hormonal pills for four years, and you’ve used them for four years, the efficacy is 91% to the fourth power. So, not at all based on research, but instead based on math.

Now I understand that math can give a good estimate but, given that this is not a peer reviewed study, it raises concerns of legitimacy and whether it’s worth considering at all. For example, if a method used typically supposedly a 50% failure rate after 5 years (not a real figure from the article, just spitballing), how many of those 50 out of 100 account for 1 of those failures, how many account for 2, etc. I feel pretty alright about my methods after not finding any studies that reflect these figures, but what about other people who might stumble upon this article? What about those prone to anxiety who stumble upon this article and think “omg! Birth control really doesn’t work! Now what?”

And to be fair, I’ve known a few people who’ve been on birth control for a long amount of time and have used it correctly and never had any failures. Anecdotal evidence is not the same as scientific evidence yes, but it does reflect just how rare pregnancy is when using methods correctly.

So, conclusion: birth control works if you make it work. Use it correctly every single time exactly as directed. Until any actual research is done on this, current typical and perfect use rates likely still apply.

[Mo]

Weird! The fact that this isn't a peer-reviewed study makes me a little concerned (as does the fact that major news outlets very very rarely represent the results of studies in a factual or reasonable way and tend towards sensationalist headlines). I am really not sure that it makes sense to use that kind of mathematical prediction in this case. But yes, I do agree with your conclusion here!! Plenty of folks use birth control methods wonderfully for a long time with no reduction in "quality" of use.