The effectiveness of ellaOne has been tested in several clinical studies and in more than 4000 women. ellaOne is not effective in every case: of 100 women receiving ellaOne up to 5 days after unprotected sex, approximately 2 will become pregnant. It means that is effective in 98%. So, as long as you use ellaOne correctly, it is very likely that it will be effective.
ellaOne should always be taken as soon as possible after unprotected intercourse, but within a maximum of 120 hours (or 5 days). You can take ellaOne at any time during your cycle.