Overdosing on fluoride supplements can cause nausea, diarrhoea and tiredness. Children aged under six should avoid using fluoridated toothpaste because they may not have learnt how to spit thoroughly. Leftover fluoride can build up in their mouths and lead to fluorosis. Fluorosis causes the development of brittle spots on teeth. This can cause teeth to break easily, which puts children at a higher risk of dental infection.