I assume you're asking about birth control PILLS here. "Birth control" is an awful lot of things -- from condoms, which you should be using even when you're on the Pill, since they are *instantly* effective against both pregnancy AND STD's, to things like Norplant, Depo-Provera, etc.
Birth control PILLS are a hormonal form of contraception. Most clinicians advise that you use a backup method of contraception for a minimum of one complete cycle of pills (1 month). Some clinicians and educators, myself included, advise that you use a backup method of contraception for 2 months (2 cycles of pills), to be on the safe side.
HOWEVER, and this is a big HOWEVER, you really should be using condoms *with* the Pill, every time you have sex. Why? First, because the Pill cannot protect you from sexually transmitted disease. Condoms can. Second, because the Pill, like every method of birth control, has some failure rate. Using the pill in combination with condoms is about the best contraception you can get, with the lowest recorded failure rate.
Be smart. Use a condom.
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