May cause teeth staining
A study was done in 2019 and it was discovered that the most popular side effect of using mouthwash is teeth staining. Some mouthwashes are made with an ingredient called chlorhexidine (CHX), which is only present if recommended.
People that use Mouthwash with bright dyes are more susceptible to teeth staining than people that don’t.
Could increase cancer risk
Some mouthwashes contain synthetic ingredients that can increase the risk of certain cancers. a study was done in 2016 that concluded that people who regularly use mouthwash may have an increased risk of head and neck cancer than people who never used it.
Experts are still studying to know if the connection is real and the ingredient that is responsible for increasing the risks of cancer.