The interaction between the combined oral contraceptives
and non-enzyme inducing antibacterials 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 oral contraceptives
(estimated at 70 million in 1996 by WHO). However, because of the personal and ethical consequences of an unwanted pregnancy, where uncertainty remains, it has been recommended that for maximal protection an additional form of contraception (e.g. a barrier method) should be used routinely while taking a short course of antibacterials (less than 3 weeks), and for at least 7 days afterwards. In addition, if fewer than 7 active pills are left in the pack after the antibacterials have stopped, the next packet should be started straight away, without a break, and any inactive pills should be omitted. Progestogen-only contraceptives
(including emergency hormonal contraceptives) do not appear to interact. Combined oral contraceptive users who start long-term courses of antibacterials need only use additional contraceptive protection for the first 3 weeks, as after that the gut flora becomes resistant to the antibacterial.
Patients who are taking long-term antibacterials for more than 3 weeks who then start
a combined oral contraceptive do not need any additional contraceptive protection,
unless the antibacterial is changed.