The time to be on birth control for it to be effective depends on the type of birth control method used:

• Combination birth control pills: If started on the first day of your period or within the first 5 days, they are effective immediately. If started at any other time, they take 7 days to become effective. Use backup contraception for the first 7 days if not started at the beginning of the period.
• Progestin-only pills ("mini-pill"): Effective immediately if started within the first 5 days of the period. Otherwise, they take about 2 days to become effective, and backup contraception is recommended during this time.
• Birth control patch and vaginal ring: Effective immediately if started within the first 5 days of the period. Otherwise, effectiveness starts after 7 days.
• Birth control shot: Effective immediately if given within the first 5 days of the period; otherwise, it takes 7 days.
• Barrier methods (condoms, diaphragm, sponge): Effective immediately when used correctly.
• Sterilization methods (vasectomy, tubal ligation): Vasectomy can take several months to be fully effective, while tubal ligation is effective immediately after the procedure.

In summary, for pills, patches, rings, and shots, starting during the first 5 days of the menstrual cycle provides immediate protection, but if started later, backup contraception is needed for 2 to 7 days depending on the method. Barrier methods work immediately when used properly.