Levonorgestrel is licensed to be used up to 72 hours after unprotected sexual intercourse. 

How it Works:  Levonorgestrel is thought to act by inhibiting ovulation, thereby delaying or preventing follicular rupture and causing luteal dysfunction. If taken prior to the start of the LH surge, it inhibits ovulation for the next 5 days, until sperm from the UPSI for which it was taken are no longer viable. In the late follicular phase, however, levonorgestrel becomes ineffective (unlike ulipristal acetate, which is still able to delay ovulation). Levonorgestrel is not effective once the process of implantation has begun. After taking levonorgestrel, women who ovulate later in the cycle are at risk of pregnancy from further UPSI. 

How Effective: Ulipristal acetate may be more effective than levonorgestrel and is licensed to be used up to 5 days (120 hours) after unprotected sexual intercourse (UPSI). The overall pregnancy rate amongst women taking levonorgestrel within 72 hours is to be about 0.6–2.6%. Levonorgestrel is unlikely to be effective if taken after ovulation.

Disadvantages: A repeated dose is required to be purchased if the woman vomits less than 3 hours after taking the tablet.  It is not effective if taken after ovulation and does not protect against sexually transmitted infections (STIs).  It does not provide ongoing contraception. There is a limited window of opportunity in which it is effective (not licensed for use beyond 72 hours after intercourse, and the evidence suggests that it is ineffective if taken more than 96 hours after UPSI).

Adverse effects may occur, such as vomiting, headache, and lower abdominal pain.

Ullipristal is licensed to be used up to 120 hours after unprotected sexual intercourse. 

How it Works: Ulipristal acetate acts by inhibition or delay of ovulation via suppression of the luteinizing hormone (LH) surge. When it is taken immediately before ovulation is scheduled to occur (when LH has already started to rise), ulipristal acetate is able to postpone follicular rupture for at least 5 days. However, it has not been demonstrated to be effective as EC when administered after ovulation. After EC with ulipristal acetate, most women will go on to ovulate later in the cycle and are therefore at risk of pregnancy from subsequent UPSI. 

Must not be used: Breast cancer; cervical cancer; ovarian cancer; severe asthma controlled by oral glucocorticoids; undiagnosed vaginal bleeding; uterine cancer

How effective: The overall pregnancy rate after administration of ulipristal acetate is about 1–2%. This means that about 1–2% of all women who take ulipristal acetate after intercourse will become pregnant, but it does not mean that a woman who has taken ulipristal acetate after UPSI just prior to ovulation has only a 1–2% chance of pregnancy. Ulipristal acetate can delay ovulation even after the start of the luteinizing hormone (LH) surge, a time when levonorgestrel is no longer effective. However, it is unlikely to be effective if taken after ovulation.

The effectiveness of ulipristal acetate could be reduced if the woman is taking a liver enzyme-inducing drug, a medication with progestogen was taken in the 5 days after taking ulipristal acetate, or in the 7 days prior to taking ulipristal acetate (theoretical risk). 

No significant reduction in effectiveness is observed with increasing time between intercourse and ulipristal acetate intake (up to 120 hours). 

Disadvantags: A repeated dose is required if the woman vomits less than 3 hours after taking the tablet.  It is not effective if taken after ovulation. It does not protect against sexually transmitted infections (STIs). It does not provide ongoing contraception. It is not suitable for women using liver enzyme-inducing drugs. Breastfeeding should be avoided for 1 week after taking. There is a limited window of opportunity in which it is effective (not licensed for use beyond 120 hours after UPSI).

Adverse effects may occur, such as vomiting, mood disorders, headache, and dizziness.