A question about Antibiotics and the Pill?

Question:

I know that taking antibiotics negates the effect of the BC pill, but for how long? Is the entire month shot or is it just during the time you're taking antibiotics?

Answers:
There are two ways in which antibiotics interact with the Pill. Some are liver enzyme inducers that increase the rate of metabolism of the estrogen in the pill resulting in reduced efficacy. Others may reduce the recycling of estrogen by destroying gut bacteria.

The recommendations by the British National Formulary are as follows;
Women requiring a prolonged course of an enzyme-inducing drug should be encouraged to consider a contraceptive method that is unaffected by the interacting drug. In women unable to use an alternative method of contraception (for rifampicin and rifabutin see also below), a combination of combined oral contraceptives should be taken to provide a daily intake of ethinylestradiol 50 micrograms or more [unlicensed use]; ‘tricycling' (i.e. taking 3 or 4 packets of monophasic tablets without a break followed by a short tablet-free interval of 4 days) is recommended (but women should be warned of uncertainty about the effectiveness of this regimen). 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.
Some antibacterials that do not induce liver enzymes (e.g. ampicillin, doxycycline) may reduce the efficacy of combined oral contraceptives by impairing the bacterial flora responsible for recycling ethinylestradiol from the large bowel. Additional contraceptive precautions should be taken whilst taking a short course of an antibacterial that is not enzyme-inducing and for 7 days after stopping. If these 7 days run beyond the end of a packet the next packet should be started immediately without a break (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; 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.

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A question about Antibiotics and the Pill?

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