I have mouth ulcer and the doctor has recommended b-complex and doxycycline and lactic acid bacillus capsules. are these medicines okay?

Mouth ulcers: What was the diagnosis?? That's the important question. The cause will determine the treatment. Was the "doctor" a physician or dentist? What you mentioned regarding medicines are fine, but will they cure or prevent the ulcers? That's a question we cannot determine over the internet without examining you and knowing your medical and dental history.