I took a zyrtec (cetirizine) at 6:30 a.m , and later at maybe 5:30 p.m. i took a plan b . will it be effective ?

Yes: No significant interaction between both drugs, be cautious though when taking multiple drugs

These two meds can: be used together. 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.