Toothpaste is designed to prevent tooth decay and gum disease while also freshening breath. Since ancient times, people have been using abrasive agents to clean plaque and stains from their teeth, but it took many thousands of years before toothpaste became truly effective at preventing decay and thus tooth loss.
All people have naturally-occurring bacteria in their mouths that attack food residue on and between teeth, creating bad breath and plaque. Plaque is a sticky material that is the major culprit in tooth decay (cavities or dental caries) and gingivitis (gum inflammation). It mostly collects around a person's gum lines, edges of fillings, and grooves on the teeth. When it is not brushed away with effective ingredients, it eventually calcifies into tartar. Both plaque and tartar contain acids that eat away the enamel surface of teeth and create cavities.
Abrasive material is the main ingredient in toothpaste. The abrasives typically are calcium carbonate or silica compounds. Emulsifiers raise residue particles off the surface of teeth so they can be more easily brushed away, and also make the paste smooth enough to be squeezed from a tube. Some toothpastes have ingredients that inhibit bacteria and prevent plaque from forming. Best whitening toothpaste contain ingredients that prevent tartar build-up.
Arguably, however, the most significant ingredient in toothpaste is fluoride, an addition which created a revolution in dental health beginning in the late 1950s.
Dentists in Italy had discovered in the early 1800s that people living in regions with high fluoride content in the soil and water had far less tooth decay, as well as an odd mottled staining on their teeth (which occurs with excessive fluoridation). Another 100 years passed before dentists noticed a similar phenomenon in Colorado. Over the next 40 years, researchers carried out extensive studies across the country and verified fluoride's unique ability to prevent dental caries.
After fluoride was added to popular toothpastes in the late 1950s (and also to community water supplies), people began seeing a tremendous improvement in the battle against tooth decay. Fluoride in toothpaste prevents the destruction of tooth enamel by inhibiting bacteria and strengthening the surface of teeth. When added to the water supply, fluoride forms a strong calcium-bonded coating on teeth, thus improving the resistance of tooth enamel. On a chemical level, it replaces the naturally occurring hydroxypatite with fluoropatite, which is less soluble and thus more resistant to decay.