Hormonal Metronidazole

The examples are aspirin, phenacetin, morphine, chloramphenicol, metronidazole, steroidal hormones etc. 

Only 3 reports of pregnancies were identified in women who took metronidazole and an oral contraceptive (unspecified) in the adverse reactions register of the CSM in the UK for the years 1968 to 1984. A survey of oral contraceptive failure identified one failure due to metronidazole (48 of a total of209 cases were attributed to antibacterials), and a follow-up study identified one fiuther case. Another survey found one contraceptive failure in a woman taking metronidazole, but she was also taking dox-ycycline (see Hormonal contraceptives + Antibacterials Tetracyclines , p.983). It is possible that these cases represent chance associations. 

The interaction between metronidazole and combined oral contraceptives is not established, and the whole issue of any interaction with broad-spectrum antibacterials remains very controversial. Bearing in mind the extremely wide use of both metronidazole and combined oral contraceptives, any increased incidence of contraceptive failure above that seen in general usage is clearly very low indeed. The Faculty of Family Planning and Reproductive Health Care (FFPRHC) Clinical Effectiveness Unit has issued guidance on the use of antibacterials with combined hormonal contraceptives. Although they recognise that there is poor evidence for contraceptive failure, they recommend that additional form of contraception, such as condoms, should be used for short courses of antibacterials, see Hormonal contraceptives + Antibacterials Penicillins , p.981, for more detailed information. This applies to both the oral and the patch form of the combined contraceptive. This advice has usually been applied to only broad-spectrum antibacterials that do not induce liver enzymes but the FFPRHC notes that some confusion has occurred over which antibacterials are considered to be broad-spectrum , and thus they recommend that this advice is applied to all antibacterials that do not induce liver enzymes, which would include metronidazole. ... 

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