Drug Interactions
Some drugs reduce the effect of the Pill and can cause breakthrough bleeding, or increased chance of pregnancy. These include drugs such as rifampicin, barbiturates, phenytoin and carbamazepine. In addition cautions are given about broad spectrum antibiotics, such as ampicillin and doxycycline, which may cause problems "by impairing the bacterial flora responsible for recycling ethinylestradiol from the large bowel" (BNF 2003).
The traditional medicinal herb St John's Wort has also been implicated due to its upregulation of the P450 system in the liver.
Read more about this topic: Combined Oral Contraceptive Pill
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