St Johns Wort is a herbal depression remedy, was quite popular here a few years back. My mother used to take it. Used to be able to get it free on the medical card if you had one but they stopped that. It interferes with some forms of BC such as the pill and implant, as far as I know it is also mentioned in a warning on the info packs. I don't know though if it would interfere with your BC straight away after a dose or if its something that builds up over a few days. I don't know anyone personally that got pregnant after it but I heard it had happened to someone at some stage. Only a test will tell for sure if you're pregnant op, I heard the dollar tests are good and first response.