“Before any dental procedure, patients must have a dental check-up first to ensure their mouth is clear of decay, gum disease, receding gums and there are no cracks in teeth or other underlying dental issues.”

Source: https://www.perthnow.com.au/news/health/perth-dentist-justina-teo-warns-of-risky-diy-teeth-whitening-c-10129397

He once had a patient with gum disease who attempted to glue their tooth back in.

Source: https://www.dailymail.co.uk/health/article-11649931/Dentist-reveals-EIGHT-things-patients-leave-horrified.html?ns_mchannel=rss&ns_campaign=1490&ito=1490

This can result in the accumulation of bacteria, increasing the likelihood of tooth decay, gum disease, and bad breath.

Source: https://www.huffingtonpost.co.uk/entry/yes-really-your-tongue-needs-brushing-too_uk_646c82b9e4b056fd46dc386f