A new drug that acts on the receptor for the hormone progesterone appears to be at least as effective as levonorgestrel (sold as “Plan B”) in preventing pregnancies after unprotected sexual intercourse, researchers report in current issue of Obstetrics and Gynecology.

“This drug (CDB-2914) represents a new frontier in emergency contraception - a single tablet with one-time dosing,” lead investigator Dr. Mitchell D. Creinin told Reuters Health. “Although other emergency contraceptive methods appear to be equally effective even with a single-dose treatment, there are suggestions that this new drug may have improved efficacy.”

Creinin of the University of Pittsburgh and colleagues came to this conclusion after studying healthy women seeking emergency contraception within 72 hours of unprotected intercourse.

In all, 775 were randomly assigned to receive a single dose of CDB-2914 plus a placebo 12 hours later and 774 were assigned to get two doses of levonorgestrel taken 12 hours apart.

Pregnancies occurred in seven (0.9 percent) of the women given CDB-2914 and 13 (1.7 percent) given levonorgestrel. Based on the estimated cycle day of intercourse, the researchers calculated that 85 percent and 69 percent, respectively, of anticipated pregnancies were averted.

Although 29 percent of the CDB-2914 group experienced nausea compared to 24 percent of the levonorgestrel group, the distribution of other adverse events was similar.

Creinin added that the agent may show advantages “compared to other products as the time from unprotected intercourse increases - specifically beyond 48 hours.”

“With currently available methods, efficacy decreases as time passes from the act of unprotected intercourse,” he concluded. “Further studies will be needed that focus on women with longer intervals since unprotected intercourse, to see if CDB-2914 truly has advantages.”

SOURCE: Obstetrics and Gynecology, November 2006.