Is plan b still effective if you are already ovulating that day?

Should work fine: Plan B can be taken on any day of a woman's cycle, so that a woman does not have to think about which day of her cycle it is. Plan B works most of the time, but not all of the time. One should do a home urine pregnancy test 3 weeks after sex. Also, use reliable forms of birth control so that Plan B will rarely be needed (hopefully never) in the future.