canker sores (aphthous ulcers) are quite common and do tend to be on the inside or mucous membrane of the mouth gums or tongue. they tend to be more-or-less isolated. herpes simplex (cold sores) inside the mouth tends to be multiple cluster-like lesions. herpetic lesions can also be on the outside of the lips. canker sores do not happen on the outside of the lips.
my partner has an ulcer on the inside of his upper lip and it looks like a canker sore but could it be a cold sore? . i had a canker sore inside my lower lip after biting it whilst eating; two days later my boyfriend got what looks like a canker sore inside his upper lip. he says he gets them when he's extremely stressed. it's white/yellow with a red ring around it but it's not uniform on the edges. his lip was swollen for a day but it's since subsided and clearly is inside the lip. could it be a cold sore/herpes and not a canker sore? he's never had a cold sore and says the mouth sores run in his family.