In a review of previous studies, researchers have concluded that the insertion of an intrauterine device after unprotected sex is the most effective and safest method of emergency contraception.

The scientists examined 42 studies published between 1979 and 2011, combining their data. When used for emergency contraception, the copper IUD has a failure rate of less than one in 1,000, making it by far the most effective method, the researchers found. Mifepristone (brand name Mifeprex) fails in about 1.4 percent of cases, and levonorgestrel (sold as Plan B One-Step) has a failure rate between 2 percent and 3 percent.

One of the studies reviewed found that despite the IUD’s efficacy and safety, 85 percent of clinicians never recommended it for emergency contraception.

According to background information in the study, published online last week in Human Reproduction, about 5 percent of women at risk of pregnancy in the United States were using an IUD as of 2008, compared with 0.7 percent in 1995.