The Food and Drug Administration on Wednesday gave the green light to a New Jersey company to advertise and sell emergency contraceptive pills that women can take after unprotected sex to prevent unwanted pregnancies.

Kits of the so-called morning-after contraceptive pills, trade- named PREVEN, will be available by prescription by the end of September, said Roderick Mackenzie, chairman and founder of Gynetics Inc. of Somerville, N.J.

PREVEN is 75 percent effective at preventing pregnancy. For every 100 women who have unprotected sex during the second or third week of their menstrual cycle, (James) Trussell explained, eight would be expected to become pregnant if they did nothing. Among those who take emergency contraceptive pills, only two will became pregnant.