The general rule of thumb is as follows: when started correctly, most hormonal birth control methods tend to be effective right away. However, although the effectiveness of most of these methods tends to begin right away, for some methods, it may take up to seven days before the contraceptive is fully effective. It is, therefore, suggested that an additional form of birth control be used, like a condom, Today Sponge, or the female condom, for the first week that a woman begins to take them -- just in case.
Likewise, if a woman starts one of these methods on any day other than the first day of her period, she will need to use another form of birth control for the first week as well.
Some doctors even recommend that women on the birth control pill use an additional form of birth control for the first month; this is especially the case for those who are taking the mini-pill. Make sure to ask your healthcare provider what he/she recommends
Finally, a woman may need to use a back-up form of birth control while she is taking various medications or herbal supplements as these can lower the effectiveness of certain hormonal contraceptives. Most doctors will suggest that a woman use another birth control method for 7 additional days once she is finished taking certain antibiotics. If a woman is unsure, she can ask her pharmacist or doctor if her prescribed antibiotic (or other medication) lowers the effectiveness of the specific pill (or hormonal method) that she is on and what actions she should take to protect herself against contraceptive failure.
Generally speaking, once effectiveness begins, hormonal birth control tends to have very high effectiveness rates: Effectiveness of Prescription Birth Control
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