Oral Contraceptives and Antibiotics

As recently as today I was told that an antibiotic prescribed for me (zithromax) would interfere with the effectiveness of oral contraception.

However, a quick scan of Medline seems to indicate that there is no evidence that most antibiotics cause OC failure above the expected rate (the exception is one called Rifampin).
see, eg, http://www.ncbi.nlm.nih.gov/pubmed/12436822

What’s the deal?

Looks like its an abundance of caution thing.


http://www.liebertonline.com/doi/abs/10.1089/104240602762555993?cookieSet=1&journalCode=gyn (low risk, abundance of caution)


I’ve heard the same thing about anti-depressants (and mood stabilisers). How does the evidence of those affecting oral contraceptive compare ?

Anecdotal evidence, but I have a good friend who became pregnant while on the pill shortly after taking a round of antibiotics. Her doctor never mentioned to her that they may reduce the effectiveness of the pill, and she was pretty pissed when he brought it up after she became pregnant. Hard to tell for sure if that was the reason, but she would have used condoms had she known there were any risk at all.

We’ve done this before.

There is little to no good evidence that antibiotics affect the effectiveness of birth control pills at all, save for some rather uncommon antibiotics like Rifampin.

Useful pdf link: http://www.jfponline.com/pdf%2F5412%2F5412JFP_ClinicalInquiries1.pdf

Just have my own personal experience. We made a baby while my wife was taking Levaquin, which seemed to make her oral contraceptives not work so well. As I understand it, Levaquin is one of the most powerful antibiotics used today. YMMV. We do love our bundle of joy, but I’d say, better safe than sorry for others.

One might ask why, if there is so little evidence that it’s an issue, there are some physicians who continue to advise caution for those using OCs when taking antibiotics.

The reason is twofold:

First, myths die hard in medicine. Once a putative association gets out there, even if completely false, every anecdotal case report comes out of the woodwork and seems to add weight to the original claim.

Second, once a myth like this gets out there, the tendency is to err on the side of caution rather than wait for additional corroboration that there is no basis for the original reports of a potential problem. It’s easier to say, “Hey, some reports say there’s an issue with antibiotics, so make sure you use alternate contraception” than it is to address someone’s Levaquin baby, even if the Levaquin baby was pure coincidence.

WTF??? What the heck is an intravenous contraceptive and where do I get some???

I belive Depo-provera is a form of intravenous contraceptive. Ask your practitioner and do a little research before trying it. It’s got some side effects.

I suppose the implants, such as Implanon, might also be considered intravenous.

I am unaware of any intravenous contraceptives for regular use. Depo shots are intra-muscular. Implants have their medications taken up on the capillary level.

Thanks, Qadgop. I’ll ask a mod to remove my post as misleadingly stupid.

While I appreciate the sentiment, we don’t edit posts for that sort of thing. I think Qadgop’s post sufficiently takes care of any misconceptions.

General Questions Moderator

The main reason, as implied, is that patients assume they can’t get pregnant because they take a contraceptive (had a tubal ligation, partner had a vasectomy)… I’ve seen lots of people get pregnant while taking OCP (and a few after surgery)…

So if someone has a Levaquin baby, is it the Levaquin or the TriCyclen? The antibiotic or the OCP?

I think its the OCP. I don’t want to saw so in court, though. In the ER I tell patients that antibiotics are not thought to affect contraception, but that contraception is not perfect.

I was taking a jab at what I thought was an obvious error, in context I’m sure the author meant to say “oral and intravenous antibiotics”

I’m thinking the late bf would’ve loved this…“my sperm is sooooo potent it cannot be stopped by a pill”

Huh. I have always gotten this warning while taking any kind of antibiotics and never heard it was not really backed by research. I often get breakthrough bleeding while on antibiotics and I just assumed it was because it was making my b/c less effective. I wonder why that is?

Good to know!

So you’re saying they would’ve gotten pregnant even if they weren’t on the anti biotic?

The are legitimate reasons why antibiotics might affect contraception.

Diarrhoea and vomiting- pill not in system long enough to work.
Negative effect on bowel flora- bacteria that is necessary to help absorb the pill from the bowel can be killed off by antibiotics.
Enzyme induction- this is how Rifampicin, HIV meds, some anti-seizure meds and St John’s Wort affect the pill’s efficacy. Simplistically, these drugs make more of the enzymes in the liver which break down the pill, so again, it isn’t in the system long enough or at a high enough dose to be effective.

My personal theory is that the sickness the antibiotics are prescribed FOR might be equally to blame:
When you’re nauseated and sick, you might not feel like taking tablets, especially if sex is the last thing currently on your mind…too bad when 5 days later you feel better and you’re unexpectedly fertile.

Oral contraceptive fail 1-5% of the time, when taken perfectly. 1 in 20 women will get pregnant on the pill, and some of them will have been on antibiotics in the preceeding weeks. You have to do quite big studies to have convincing enough data to prove that antibiotics aren’t a factor once confounding factors have been outruled.

As a doctor saying “take extra precautions while taking these tablets and until your next period” won’t do any harm and is unlikely to get you sued, but knowing there is a theoretical risk of contraceptive failure and not mentioning it to a patient who then falls pregnant…not as easily defendable.

Have they done studies showing a reduction in BC effectiveness with St. John’s Wort? Back when I was last actively educated in herbalism, it was a theoretical link only, based on suspected metabolic pathways, with no studies showing a problem in actual people. The common wisdom was that, as there are so many women in Germany taking SJW and oral contraceptives, and their BC failure rate is no higher than anyone else’s, it was probably not a real risk. But I’ve been out of the loop for nearly 4 years now, so I don’t know if there’s been anything done recently.

It’s in my BNF as a possible drug interaction, can’t be bothered to look it up to see if there are studies, sorry, I’m just too lazy. But SJW is known to induce liver enzymes, so it’s safer to take the cautious route with it.

There are certainly legitimate reasons antibiotics might affect OCP, and Irishgirl did a good job of saying what they are.

But if studies show no significant effect, that is that the failure rate of OC remains the same for most people taking most antibiotics, than these legitimate reasons are probably not true for most people. If OCP failure rates are 1-5%, most of the time I do think the pregnancy would have happened anyway. Antibiotics can cause nausea and vomiting, but usually don’t, and people may feel less amorous if these symptoms are severe.

The P450 research on enzyme induction is interesting, and sometimes very important – but it’s often hard to know when. I can’t remember, as a doctor, telling a patient with prostate problems not to eat barbecued hamburgers.