Maximizing the effectiveness of 1.5 mg levonorgestrel for emergency contraception: The case for precoital use

U.S. and World Health Organization Selected Practice Recommendations for Contraceptive Use state people may have an advanced supply of emergency contraception (EC) to minimize treatment delays. We sought to characterize the potential improvement in effectiveness of 1.5 mg levonorgestrel (LNG-EC) if it were taken up to a few hours before unprotected sex.

We expanded on an existing mathematical model for the maximum attainable effectiveness of LNG-EC, assuming it exclusively works to disrupt ovulation, and compared results with point estimates from nine studies when it was taken up to 72 hours after sex. We then modelled how effectiveness might have improved if subjects had taken LNG-EC up to 3 hours before sex.

Taking LNG-EC immediately after sex could potentially reduce the risk of unintended pregnancy by 91%. However, population-average maximum attainable effectiveness levels ranged from just 49% to 67% when accounting for the distributions of postcoital treatment delays in the example studies. If half the subjects had taken it 3 hours before sex, then maximum effectiveness levels would have ranged from 70% to 81%.

At the individual level, taking LNG-EC a few hours before sex is a logical extension of Selected Practice Recommendations regarding an advanced supply of EC and, based on our modeling, should be advocated for people who can reasonably anticipate an unprotected sex act. In the absence of more clinical data, however, people should not routinely rely on precoital use of LNG-EC to prevent pregnancy unless modern, effective contraceptives are inaccessible to them.

Based on mathematical modeling, individuals who anticipate needing to take LNG-EC for an impending unprotected act of sex could further reduce their chance of an undesired pregnancy by taking it a few hours in advance.