Penicillins Contraceptives, combined hormonal

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. ... 

The interaetion between combined hormonal eontraceptives and penicillins is inadequately established and controversial. Almost all of the evidence is anecdotal with no controls. The total number of failures is extremely small when viewed against the number of women worldwide using combined hormonal contraceptives (estimated at 70 million in 1996 by WHO ), so most women are apparently not at risk. 

Note that the mechanism behind the rare cases of failure of combined oral contraceptives seen with various broad-spectrum antibacterials is postulated to be reduced enterohepatic recycling of ethinylestradiol (see Hormonal contraceptives + Antibacterials Penicillins , p.981). Since progestagens are largely metabolised to inactive substances before they are conjugated, they do not undergo enterohepatic recycling of the active substance. 

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