Hello and welcome to health care magic forums.
Mouth ulcers are not a common side effect of birth control pills, especially the new generation ones that we use today.
Even then, every person's body is unique and sometimes we can develop rare side effects with certain drugs.
If the rate of ulcer formation are so high, then take her to an OBGYN
specialist along with the pill she takes. The doctor will prescribe a different class of pill. Moreover she will also be given a course of Vitamin tablets since Vitamin deficiency
is a chief cause of oral ulcers.
Do add a lot of citrus fruits in her diet.
The ulcers can be stress
related as well.;so make sure she is happy and calm.
Thank you for consulting us.