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Welcome to Just Answer and thanks for your question. I frankly think Plan B would be an overreaction. There is always a minor chance that sperm in pre-ejaculate will enter the vagina, the odds aren't that great. Of course, there is never 0% risk, unless, of course, you are on birth control.
Please let me know if you have questions.
Plan B will interfere with the scheduling of your normal period for the next couple of months. That is the biggest downside. It is very, very effective if taken within 48 hours of unprotected intercourse.