It is caused by a lack of B vitamins, but that said, infection is probably also a cause. Some people don't have the bacteria that make some of the B vitamins and vit K in their guts so may have to take some supplements. This is the FIRST sign of a deficiency. Imagine how bad it could get.
Angular Cheilitis:
A deficiency of B2, Riboflavin. Very common on a modern Western diet. Don’t be a vegetarian! Don’t drink any kind of soda, cola, fizzy pop drinks. Take 1 x B2 + 1x multi-B tablet for a week, (!00 to 200mg) Recommended dose is 1.5 to 2 mg, this is much too low for young people. 100mg once a week thereafter or 10mg daily.
Source: Experience, Merck’s manual, Upjohn’s vitamin manual, the Nurient bible by Henry Osiecki. Newton’s pharmacy.
NHMRC riboflavin recommendations for Australian adults: