@john -- I am no doctor, and yes there is always a risk.
Check this link out:
www.ncbi.nlm.nih.gov/pubmed/7614202
www.ncbi.nlm.nih.gov/pubmed/15811049
I always use Listerine before and after a booking. Whether it really protects you I don't know, but it is better than doing nothing I guess.
According to this study it is effective though against some stds and other infections.