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If you have been regular on them (the previous pack), then they will be effective as soon as you start with the next pack. Continue to take them regularly without missing pills in between for complete efficacy.
Report to your healthcare provider/gynecologist if you miss more than 1 pill anytime during the pack as it will not be effective in such cases. You will need to use back-up method of birth control along with taking the pills.
Take care.