Although Plan B has been proven effective, no one is 100 percent positive about the mechanism of the drug, she said.

"The primary theory is prevention of ovulation," she said.

If the release of an egg is delayed during ovulation, fertilization and pregnancy will not occur, said Shelley Haffner, a nurse manager at University Health Services.

"Progesterone is produced once ovulation occurs," Haffner said. "The high dose of progesterone tricks the body into thinking it has a high level of progesterone and that ovulation recently occurred."

The women's body then delays the release of another egg because of the elevated levels of progesterone, Haffner added.

In normal fertilization, an egg is released from the ovary and fertilized in the fallopian tubes, Moyer said. It then travels into the uterus, where the lining of the uterus prepares for implantation, she said.

If an egg has already been released and is fertilized in the fallopian tubes, Plan B can disrupt the cells lining the uterus, preventing implantation of the fertilized egg, she said.

Although some people believe Plan B to be an abortion pill, she said she believes it is not.

"If a pregnant woman takes Plan B, nothing will happen to her pregnancy," Moyer said.

An abortion pill interrupts and terminates an already established pregnancy, but Plan B prevents the pregnancy, she said.

A pregnancy cannot be detected in the first few days of fertilization, she added.

Plan B consists of two pills that she advises to take at the same time, despite the packaging stating 12 hours between pills.

"Sometimes people aren't compliant with the second dose, and taking two at once eliminates worrying," she said.

She said she is only aware of one side effect: nausea.

Plan B has been proven to be very effective, she said.

"If women in their second or third week of a cycle [have unprotected sex], eight out of 100 would become pregnant," she said. "But if all 100 took Plan B, only two would become pregnant."