I recently got on birth control, I waited a week until I had unprotected sex so the pill could adjust start taking effect.

But I'm still a little scared because I had sex without a condom after the week and I'm just paranoid. So can someone tell me how effective this pill is and the percentage of pregnancy risk and any other facts I need to know about the pill?


2 Answers 2


In essence there is a very good answer already provided here: https://skeptics.stackexchange.com/questions/28942/does-taking-birth-control-pills-make-you-99-9-protected-from-pregnancy

The effectiveness of the "pill" in terms of birth control will depend largely on how accurately you are following the advice given by your doctor / pharmacist on how and when to take it.

Technically, if you are following the procedure correctly, it should be a very effective method of birth control and works in up to 99.9% of cases: http://www.webmd.com/sex/birth-control/birth-control-pills

Now, that is not 100% and certainly there are different studies out there, which will provide different results. Essentially, what this means is that you can still get pregnant this way. I don't want to scare you and the probability is low, but you should know that it is still possible. If you forget to take it one day, the probability obviously increases ...


You also absolutely need to know that the "pill" does not protect you from any sexually transmittable diseases! I trust you know this and your partner knows this, too.

A little off-topic:

If either of you hasn't gotten a HIV test - don't be naive - go and get it done! You need to know that it can take 3 months before this blood test may reliably detect HIV, so if either of you had a different partner in the last three months, it's not worth it - don't risk it! You need to be sure about this. The number of new HIV infections is actually increasing in the western world, even though free HIV tests are available to many. This is true all across the US, Australia and Europe: http://www.euro.who.int/en/media-centre/sections/press-releases/2015/11/highest-number-of-new-hiv-cases-in-europe-ever

  • The link you provided to support your 99% figure actually says 95% (after I managed to drill through the ads and found the info). And half your answer is a sermon on HIV. She didn't ask about HIV or sexual counseling.
    – Carey Gregory
    Mar 11, 2016 at 6:27
  • Sorry, but you are not correct. Quote from the website: "Women take the pill by mouth to prevent pregnancy, and, when taken correctly, it is up to 99.9% effective." The question also clearly extends beyond the pill: Quote: "and any other facts I need to know about the pill"
    – Chris
    Mar 11, 2016 at 7:29
  • @CareyGregory You are constructing misleading commentary that is simply incorrect. The only reference on that page regarding a 95% figure is related to a "mini pill", which is explicitly referred to as "less effective than the standard pill", which is clearly what this was about.
    – Chris
    Mar 11, 2016 at 7:33
  • @CareyGregory It's really repulsive to everyone involved if you desperately try to find something wrong with an answer - if you don't want people to exchange knowledge here, then please don't participate.
    – Chris
    Mar 11, 2016 at 7:36
  • 2
    I'm citing the figures found at the link you provided. Unfortunately, your link is to WebMD, which is a poor source, and it's not to a specific topic but to a general discussion.Therefore, I had to wade through advertising to find the info on my own, and the first info I found was for the mini pill. In fact, I don't see a link to effectiveness for standard pills anywhere on that page. Also, no other source I can find gives birth control pills the 99.9% effectiveness you claim so you really do need to provide a definitive source for that number.
    – Carey Gregory
    Mar 11, 2016 at 15:16

If by "I waited a week" you mean you waited seven days, you should be fine. However, seven days is the bare minimum so if you mean anything less than a full seven days then it depends on when you began to take them relative to your menstrual cycle. These are typical instructions for beginning birth control pills:

  • Start your first pack of pills on the first day of your menstrual period. Use a backup method of birth control (such as condoms with spermicide) until you start the pill. The pill will start working right away for birth control if you start on the first day of your period.
  • Start your first pack of pills on the Sunday after you start your menstrual period. You should use a backup method of birth control (condoms with spermicide) until you start the pill. You also will need to use a backup method of birth control (condoms with spermicide) during the first 7 days of the pill package.
  • Start your pills today. If you start your pills today, you should use a backup method of birth control (condoms with spermicide) during the first 7 days of the pill package. Birth control pills will start to protect you from pregnancy after the first week, if you take them correctly.


Another answer cites a source that gives a very high effectiveness number. WebMD states that "when taken correctly, it is up to 99.9% effective". However, there are a couple of problems with that number:

First, WebMD doesn't provide a source for the number, and given that an ad for Bayer birth control pills appears directly next to it, I would have to assume that the number came from Bayer's advertising department rather than an objective scientific source.

Second, the language WebMD uses is very important here. They state that it is "up to 99.9% effective" when "taken correctly." Well, that's quite a bit like the gas mileage numbers automobile manufacturers state for their cars. They obtain those figures using professional drivers under strict laboratory conditions and, while accurate, virtually no consumer will ever achieve those same results because they are real people driving on real roads, not professionals driving on closed tracks. Likewise, in the real world women do not achieve 99.9% effectiveness with birth control pills. According to the US Centers for Disease Control, the effectiveness of birth control pills actually observed in the real world is 91%. Other authoritative sources such as the US Department of Health & Human Services, Planned Parenthood, et al. cite the same 91% figure. The 99.9% figure cited by WebMD is in fact obtained from perfect compliance during clinical trials. (And yet it's still incorrect -- the actual figure is 99.7%.) As you might imagine, patient instructions and supervision of compliance during clinical trials is much stricter than it is in the real world of medicine. Perfect compliance in the real world is actually a rare thing. So you should assume going forward that you have about a 1-in-10 chance of becoming pregnant over the course of a year if you use birth control pills as your sole source of protection.

If you would like to consider alternatives to the pill in the future, the CDC has published a very simple comparison of the effectiveness of various options.

Other Facts I Need to Know

Some drugs reduce the effect of the pill and can cause an increased chance of pregnancy. It's probably a good idea to consult your doctor or the pharmacist who fills your prescription when you're given a new prescription.


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