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First of all it is not advisable to take pills to get periods.
A negative pregnancy
test does not exclude pregnancy completely but only suggests that the pregnancy if present is not of the duration which can be detected by that particular test.
There have been hundreds of thousands of cases where the pregnancy test was negative and the person did not get the periods after taking the pills to get period and subsequently it was found that the person was indeed pregnant and this gives rise to anxiety
regarding the safety of these pills in pregnancy.
If you are 100 % not pregnant, then the withdrawal bleeding should occur within 7 days.