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An oc -pill pack is formulated based on a standard 28 day cycle of a lady . so even if you miss your 21st day pill , your withdrawal would occur in next 7 days and then a fresh pack have to be started from day 1 of next cycle.
Unlikely there would be any side effects per se and there would be no change n the cycle either, but you would stand unprotected early by a day.
i hope i have answered your query,
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