Is having sex after taking first dose of plan b okay and will it serve as a contraceptive?

Nope!: The contraceptive effect of Plan B (levonorgestrel) is probably not as long as the lifespan of sperm in your genital tract! you should use a back-up form of birth control (such as condoms) to prevent pregnancy after taking plan b. In addition, you might discuss with your doctor a more long-acting and effective method like an intrauterine device if you are serious about trying to prevent pregnancy.

No: No, Plan B is only an emergency contraceptive method to use when the condom broke or on victims of sexual assault to prevent pregnancy if taken at the right time, but you need a back up method also to make sure you are covered for the rest of the time.