I went to the doctors office two days ago because i hav a very bad sore throat. about a few weeks ago i took a pregnancy test and it was neg, i am on birth control and thought that the test results were wrong. i asked the doctor and he told me that you should get valid answers on your pregnancy test even if your on birth control.
I asked the pharmasist about this one once. She said false negatives are possible because of the pill, false POSITIVES are rare.