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Plan B is very effective at preventing (not terminating) pregnancy if taken soon after intercourse.
That doesn't mean it's all fun and games to take. The women I know who have used Plan B have all experienced intense cramping, stomach pain / nausea, and general feeling-icky-ness, comparable to the WORST period a female has ever had. (But again, well worth avoiding an unwanted pregnancy.)
Believe me, the women I know who experienced Plan B once were smart enough to take better preventative measures next time, to avoid feeling that ill.
Anecdotally, some college health centers address the issue of female students who come in for repeat Plan B prescriptions. When women need it more than once in a short time span, they and partner are mandated to undergo safe-sex training (including the condom-on-the-banana) to increase the likelihood that they won't need Plan B if their chosen Plan A is being used correctly.