Smokers have an increased chance of experiencing cold symptoms and mouth ulcers for several weeks after quitting. Mouth ulcers seem to be more prevalent in more dependent smokers. The ulcers resolve within 4 weeks in 60% of people. Mouth ulcers following smoking cessation may be related to the absence of an antibacterial effect of smoking, change in the keratinization of the mouth tissue when quitting or change in one's immune response. They are not thought to be related to NRT use, however the mechanical chewing or sucking of NRT may irritate the ulcers. NRT has actually decreased the severity of ulcers in some ex-smokers. Cold symptoms might be caused by a temporary reduction in a salivary immunoglobulin upon quitting.
There are many OTC and prescription products that can alleviate the symptoms of the mouth ulcers. Talk to your pharmacist and/or doctor about these.
Hayden M, et al. The relationship between smoking cessation and mouth ulcers. Nicotine & Tobacco Research 6(4): 655–659.