Will taking valtrex for cold sores outbreak on mouth area affect the efficacy of combined oral contraceptive (yasmin)?

Probably not: The current thinking is that except for rifampin, other commonly-used anti-infectives do not alter the effectiveness of birth control pills. This is likely true, presuming a person is not sick with vomiting or diarrhea, or some other condition that decreases the absorption of a swallowed pill. One can look up: http://www.ncbi.nlm.nih.gov/pubmed/12063491