Is Plan B still effective after 72 hours?

Could be. If your talking like not long after 72 hours. You could try it up to 4-5 days, it won't cause a birth defect or an abortion, it changes the uterine lining to avoid implantation.
Much less effective. 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.