I received oral sex and I noticed when the person spit afterwards there was blood in their spit. If they are HIV positive aren't I at risk? I was on Truvada for prep for 12 days but I read it takes 7 days to be effective against receptive anal but 20 days for insertive anal, insertive vaginal, and against needles. I can't find any info on how long prep takes to be effective against receptive oral. How long does it take for prep to be effective against receptive oral? Lastly I read prep for 28 days after exposure works as PEP. Is this true or do I need to get more than Truvada for PEP as some sites say?