Antibiotics such as amoxicillin, ampicillin, erythromycin and tetracycline affect the re-absorption of oestrogen. These antibiotics alter the gut flora and ethinyloestradiol is not conjugated. There is more ethinyloestradiol passed in the stool. Pregnancy and breakthrough bleeding can occur

• if any of course of broad-spectrum antibiotics is taken then the woman should continue to use the pill but should also use a barrier method whilst taking the antibiotics and for seven days after stopping. If these seven days extend to the pill-free period then omit this (2) (in the case of ED tablets the inactive ones should be omitted)
• if the antibacterial course exceeds 3 weeks, the bacterial flora develop antibacterial resistance and additional precautions become unnecessary unless a new antibacterial is prescribed (3) - therefore if a woman is already taking the combined oral contraceptive and commences prolonged antibiotic treatment then additional precautions are required for the first 3 weeks only
• additional precautions are also unnecessary if a woman starting a combined oral contraceptive has been on a course of antibacterial for 3 weeks or more

Notes:

• in the case of acne vulgaris an alternative therapeutic option would be the prescribing of a form of contraception with additional treatment properties such as Dianette (R)
• the guidance above is based on the use of antibacterials that are not enzyme-inducers (e.g. rifampicin, rifabutin)
  • rifampicin and rifabutin are such potent enzyme-inducing drugs that an alternative method of contraception (such as an IUD) is always recommended. Since enzyme activity does not return to normal for several weeks after stopping an enzyme-inducing drug, appropriate contraceptive measures are required for 4 to 8 weeks after stopping (3)

Reference:

1. Russell-Jones R.Guidelines for GP Referrals in Dermatology. Magister, 1998.
2. Doctor (March 2005). Ready Reckoner - Contraceptive efficacy.
3. BNF 7.3.1