Trendy social media influencers are promoting the recipes amid growing mistrust of ingredients used by industry. But a study of 84 homemade concoctions found on the internet reveal they are likely to do more harm than good. They included the likes of charcoal, clay and turmeric, which can contain toxic heavy metals. Others contained abrasive lemon juice, with an acidity level that can strip teeth of protective enamel and increase the risk of damage. None contained fluoride — the key ingredient for preventing decay — and few contained foaming ingredients, preservatives or antibacterials. It means they could become contaminated with harmful bacteria. Researchers from the University of Nantes, France, also express concern that the recipes use imprecise measurements, such as tablespoons and teaspoons.