"About 16% of women report side effects like nausea, vomiting, or irregular bleeding," says Dweck. But the main downside is that it can cause a lot of menstrual irregularity. Depending on how often you take the morning-after pill, it can throw off your cycle for weeks or even months, says Streicher. And a messed-up cycle can make it harder to prevent pregnancy if you aren't using another method like condoms.
"You never know for sure when you are ovulating, even less so if you have taken a big dose of hormones that may interfere with your normal cycle," Streicher says. So you should always remember to use a backup barrier method when you're having sex after taking emergency contraception.
Plus, it's a lot of unnecessary hormones when you just could be on a more reliable, low-dose method. For instance, if you have unprotected sex 20 times in one month (and take Plan B 20 times in one month), that's a lot of progestin to take as opposed to taking a small, regular dose every day — not to mention a lot of spotting or bleeding.