All antibiotics, including levaquin, lessen the effectiveness of the pill. They do so because they cause diarrhea. Diarrhea can cause the pill to be flusehd from your system before it has been properly digested and absorbed. For instance, if you have diarrhea and the contents of your stomach are passed into the intestines when only half the pill has been absorbed, the pill is only half as effective as it would normally be. As long as you are on the antibiotic you need to use a backup method of birth control. In addition, you need to use a backup method for ANOTHER 7 days after you are done with the antibiotic in order for the pill to build back up in your system and become fully effective again.
It's a pain but this is one of the instances where you need to be extra careful with birth control.