Is a negative pregnancy test taken after period is 8 days late accurate? took plan b and second period is late.

Likely accurate: Potential Side effects w emergency contraception include menstrual flow changes (spotting, bleeding, early or late period). Take pregnancy test & notify Dr if your period is 7 or > days late.

Maybe (not): It depends on how soon after the encounter you took the Plan B (with 72 hours). The hormones from the Plan B may have disrupted your cycle a bit too. A pregnancy test if your period doesn't come soon would be wise.