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How long does it take for birth control to become effective after switching brands?

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I recently switched my bc because I was bleeding a whole week before my period started. I was taking ortho-tri cyclin for 3 months and this Sunday I'm starting my 4th pack. The 4th pack is the new brand. I know it takes about 3 months for bc to become effective but now I'm not sure since I'm switching if I have to wait another 3 months or is it automatically effective since I never missed a day since I started?