Potential side effects of using mouthwash include staining of the teeth and oral appliances, irritation of the mouth and gums, and lesions of the inside of the mouth, according to the Pharmaceutical Journal. Side effects are most common with antibacterial and tooth-whitening mouthwashes.
Side effects from plaque-inhibiting and fluoridated mouthwashes are infrequent and usually mild, according to the Pharmacy Journal. Though some plaque-inhibiting mouthwashes contain relatively high amounts of alcohol, research indicates no link between regular use of these mouthwashes and oral cancers. However, mouthwashes containing the antibacterial agent chlorhexidine can cause acute lesions of the oral mucosa in sensitive individuals.