Taking Keflex (Cephalexin) With Birth Control

The pharmacist discusses whether or not Keflex (cephalexin) interacts with or reduces the effectiveness of birth control.


My Dr. put me on 500mg of cephalexin 3 times a day and 150mg of fluconazole. I am currently taking Estarylla birth control pills. I'm terrified that the medication my Dr. gave me will interfere with the birth control and I will end up pregnant especially with all the controversy about antibiotics and birth control pills. My Dr. said I would be fine if I had unprotected sex but I'm not so sure. So is it really ok if I have unprotected sex?

Asked by Kenzie On Jun 14, 2018

Answered by
Medical Content Reviewed By PharmacistAnswers Staff

On Jun 15, 2018

Birth Control PillWhether or not antibiotics, like Keflex (cephalexin), reduce the effectiveness of birth control is uncertain. Most data suggests that the vast majority of antibiotics do not significantly reduce birth control effectiveness, with the exception being the rifamycin family of antibiotics.

Nevertheless, as the consequences of unintended pregnancy are significant, and there isn't conclusive data regarding just how Keflex may affect hormone concentrations in birth control in all women, it would be prudent to use back-up contraception when taking the antibiotic.

Keflex Interaction With Birth Control

Cephalexin Splash

Historically, it was thought that antibiotics could reduce the effectiveness of birth control pills by altering the bacterial flora in our gastrointestinal tract, which was thought to decrease overall estrogen levels. The mechanism behind this interaction is as follows:

  • The estrogen (ethinyl estradiol) in oral contraceptives is primarily metabolized in the liver to the metabolite 2-hydroxy-ethinylestradiol and other hydroxylated and methylated metabolites.
  • These estrogen metabolites undergo what is known as enterohepatic recirculation. This means that these estrogen metabolites are recirculated into the GI tract after being processed by the liver.
  • The bacteria in our GI tract can change the metabolized estrogen back into the active, parent compound, ethinyl estradiol, which increases overall absorption and concentrations of estrogen.
  • Antibiotics, like Keflex, theoretically eliminate much of our normal GI flora. Therefore, less of the metabolized estrogen is able to be converted back into the active compound, reducing overall concentrations and protection from pregnancy.

While the above may be the mechanism behind the potential antibiotic-birth control interaction, it doesn't appear be significant for most people.

What Do The Studies Say?


The vast majority of studies on the matter conclude that most antibiotics do not significantly alter the concentrations of hormones from birth control pills and therefore do not reduce the effectiveness of birth control. Antibiotics that have little evidence of this interaction include:

  • Ampicillin
  • Penicillin (conflicting data)
  • Cephalexin
  • Ciprofloxacin
  • Clarithromycin
  • Doxycycline
  • Metronidazole
  • Tetracycline (conflicting data)

The only antibiotics that have been definitively shown to decrease hormone concentrations are anti-tuberculosis drugs like rifampin, which interact with oral hormones via an entirely different mechanism (by speeding up metabolism of estrogen).

However, and this is a big however, one large study, published in Obstetrics and Gynecology, reported that a small number of individual patients, who are difficult to accurately identify, did experience significant decreases in hormone concentrations from birth control pills when taking antibiotics and these individuals ovulated (which birth control pills are supposed to suppress). 

The study concluded the following:

"Pharmacokinetic studies of other antibiotics have not shown any systematic interaction between antibiotics and OC [oral contraceptive] steroids. However, individual patients do show large decreases in the plasma concentrations of ethinyl estradiol when they take certain other antibiotics, notably tetracycline and penicillin derivatives. Because it is not possible to identify these women in advance, a cautious approach is advised."


So overall, if you aren't taking a rifampin type antibiotic, which Keflex is not, the effectiveness of your birth control will probably not be significantly affected. However, as there may be some individuals more susceptible to the interaction, and because unplanned pregnancy has life changing consequences, back-up birth control (e.g. condoms) is recommended.

In terms of how long to wait after taking antibiotics to stop back-up contraception, the most common recommendation is for 7 days after you finish your antibiotic. This allows time for the bacteria in your gut to "re-grow" and for hormone concentrations from the birth control pills to return to a normal level.

About the Pharmacist

Dr. Brian Staiger Pharm.D

Dr. Brian Staiger is a licensed pharmacist in New York State and the founder of PharmacistAnswers.com. He graduated from the University At Buffalo with a Doctor of Pharmacy degree in 2010. He has been featured in numerous publications including the Huffington Post as well as a variety of health and pharmacy-related blogs. Please feel free to reach out to him directly if you have any inquiries or want to connect! He's answered thousands of medication and pharmacy-related questions and he's ready to answer yours! Brian.Staiger@PharmacistAnswers.com Office: 716-389-3076

About Keflex (Cephalexin)

Keflex (cephalexin) is a first-generation, cephalosporin antibiotic and is mostly used for the treatment of gram-positive bacteria. Keflex is one of the most widely prescribed antibiotics, used for the treatment of otitis media (ear infection) and infections of the respiratory tract. Keflex may be given with or without food. However, food may decrease nausea and other GI related symptoms. Keflex is generally dosed 2 to 3 times daily.

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