Impact of Antibiotics on Birth Control Pills: What You Need to Know

Interactions between contraceptive pills and antibiotics?

Hello, doctor.
My girlfriend was in a car accident on October 3rd, and the hospital prescribed Cephalexin 500 mg, four times a day for three days.
She finished the medication on October 6th, and on the same day, she started taking the contraceptive pill for the first time, as her period began.
On the night of October 14th, we had unprotected intercourse.
I would like to ask if this affects the effectiveness of the contraceptive pill.
Additionally, after stopping other medications, does she only need to take the contraceptive pill for seven days to ensure its effectiveness? Thank you for your response, doctor.

Andrew, 20~29 year old female. Ask Date: 2020/10/15

Dr. Zhang Kunmin reply Obstetrics and Gynecology

Hello, the antibiotics that may interact with oral contraceptives include Penicillin, Cephalosporins, Erythromycin, Sulfonamides, Bactrim, and Chloramphenicol.
If the timing of administration does not overlap, most of these medications are metabolized.
However, if there is no expected withdrawal bleeding during the interval when no pills are taken, pregnancy should be suspected.
Thank you! Wishing you peace and health, Dr.
Chang.

Reply Date: 2020/10/15

More Info

The concern regarding the interaction between antibiotics and birth control pills is a common one, and it's important to address it with accurate information. In your case, your girlfriend was prescribed Cephalexin, a type of antibiotic, after her car accident, and she began taking her birth control pill (in this case, a combined oral contraceptive) shortly after finishing the antibiotic course.

First, it’s essential to understand that not all antibiotics affect the efficacy of birth control pills. Most antibiotics do not interfere with the effectiveness of hormonal contraceptives. However, there are exceptions, with the most notable being Rifampin and Rifabutin, which are used to treat tuberculosis and some other infections. These specific antibiotics can reduce the effectiveness of hormonal contraceptives by increasing the metabolism of the hormones in the liver, leading to lower levels of these hormones in the bloodstream.

Cephalexin, on the other hand, is a cephalosporin antibiotic that is not known to have this effect on birth control pills. Therefore, taking Cephalexin should not have compromised the effectiveness of the birth control pill your girlfriend started on October 6, especially since she completed her antibiotic course on the same day she began taking the pill.

Regarding your question about the timing of protection from the birth control pill, it is generally recommended that for combined oral contraceptives, if a woman starts taking the pill during the first five days of her menstrual cycle, she is protected against pregnancy immediately. Since your girlfriend started taking the pill on the first day of her period, she should have been protected from the outset.

If she had started the pill at a different time in her cycle, it would typically take seven days of consistent use for the pill to reach full effectiveness. However, since she started on the first day of her period, she does not need to wait for the full seven days to be protected.

In summary, based on the information provided, the use of Cephalexin should not have affected the efficacy of the birth control pill, and your girlfriend was protected from the first day she started taking it. It’s always a good practice to consult with a healthcare provider for personalized advice, especially when it comes to medications and contraceptive methods. If there are any concerns about missed doses or potential interactions with other medications in the future, it’s best to discuss these with a healthcare professional to ensure continued protection and health.

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