The "morning-after pill" [1] refers to a class of medications that can be taken after unprotected sex to prevent pregnancy. While there are several brands, there are only two types of single-dose pills that can be used as emergency contraception, both of which can cause side effects [2]. Here's what you can expect for each and how to know if what you're experiencing merits a call to your doctor.

Progestin-Only Emergency Contraceptive Pill

Progestin-only emergency contraception, sold under brand names like Plan B and Next Choice One Dose, can be taken within three days of unprotected sex. These pills work to prevent pregnancy by delaying ovulation and thickening the cervical mucus, which acts as a barrier to sperm.

The most common side effects are nausea, abdominal pain, irregular bleeding, headache, and fatigue. Though vomiting isn't as common with progestin-only options as it is combined pills — you can learn more about the risks and benefits of different emergency contraceptives [3] here — if it does occur within the first hour of taking the pill (some even say two hours), you may need to take another dose in order to ensure that the medication is effective.

The pill may also cause changes to your menstrual cycle, with your period arriving earlier or later than expected [4]. (If your period is late by a week or more, you could be pregnant and should consult your doctor.) The amount of time that the medication will remain in a person's body has not been well-studied, but in my practice, I've never had patients report side effects for more than one month.

Ulipristal Acetate Emergency Contraceptive Pill

UPA is most commonly sold under the brand name Ella and works to delay ovulation by blocking the progestin receptors in the body. Unlike the progestin-only pill, it can be taken up to five days after unprotected sex, though you'll need a prescription. Your doctor can determine if UPA is right for you based on your medical history and any medications you're taking, including birth control.

Common side effects are similar to those listed above, with abdominal pain being slightly more common. On average, those taking UPA will have their period two days later than expected [5]. Again, the duration of side effects hasn't been well-studied; however, if you're still experiencing abdominal pain three to five weeks after taking the pill [6] (particularly if it's severe), it's important to see your doctor.

The bottom line is that side effects after taking emergency contraception are common, but don't hesitate to contact your healthcare provider if you're concerned about your particular symptoms or if they seem to deviate from what is expected. It's always best to have the conversation.