A cold sore is a small painful blister on or near the lips, or sometimes the nose, caused by the virus Herpes simplex type-1 (HSV-1). Nitric oxide increases blood circulation in the muscles which is important for supplying the muscles with nutrients in order for them to work and build muscle mass. I'm unable to produce any data about nitric oxide's influence on people with HSV-1 infection but you might consult the producers of the supplement to ask why the supplement is contraindicated for people with HSV-1.