Many toothpastes have fluoride in them to help protect your teeth against cavities. It is believed that fluoride does this by inhibiting the breakdown of the tooth surface, while at the same time promoting remineralisation of the surface.
The toothpaste that is kept behind the counter is there because it has a high concentration of fluoride in it. So why not let everyone have the higher concentrated toothpaste? Because there is a syndrome known as fluorosis.
In its mild form, fluorosis appears unnoticeable, as tiny white streaks or specs on the tooth. In its most severe form it can cause discolouration and brown markings on the tooth. It can even cause the enamel to be rough, pitted, and hard to clean. Unfortunately these spots and stains are permanent and can darken over time.
Fluorosis normally occurs when people are exposed to high levels of fluorosis over a long period of time e.g. from food, water, air, and excessive use of toothpaste. While acute toxicity is possible (and can cause nausea, vomiting, seizures and muscle spasms) it is uncommon as acute high-level exposure is rare.
Interestingly, concerns for fluorosis is the reason why parents are advised to make sure their children only use a pea size amount of toothpaste to brush their teeth, and to ensure they do not swallow the toothpaste.
World Health Organisation, Water-related diseases, Fluorosis