How effective is plan b after 27 hours?

Quite effective: Emergency contraception (EC) pills are used after sex to lower the chance of pregnancy by about 70-80%. EC is a high dose of birth control. Abnormal bleeding or spotting is a common side effect. Nausea, abdominal pain or cramping, and fatigue may occur, but symptoms resolve in a day or two as the drug leaves the body soon. Women may also get breast tenderness, headache, dizziness, or vomiting.

Plan B: Plan B One-Step is up to 89 % effective when taken within 72 hours (3 days) after unprotected sex. It can reduce chance of pregnancy by up to 95% if used within 24 hours. It continues to reduce the risk of pregnancy up to 120 hours (5 days) after unprotected sex, but is less effective as time passes.