You rinse at the end and send the fluoride down the drain

In Scotland, in a trial that ran from 1988 to 1992, more than a thousand schoolchildren were asked what you did this morning: whether they rinsed their mouths with water after brushing. The ones who rinsed had more decay than the ones who only spat, at the same brushing frequency. It was an observational analysis inside that Lanarkshire caries trial, published in 1998, so on its own it does not prove a cause; what it does is point in the same direction as what gets measured in the laboratory.

This morning you brushed your teeth for a good two minutes, you spat, you turned on the tap and you rinsed your mouth out until it tasted of nothing at all. Those last three seconds are the ones that undo the rest.

Nobody explained it to you because toothpaste is not soap. It does not work by scrubbing: it works because it leaves fluoride stuck to the surface of the enamel and dissolved in your saliva for the hours that follow. That fluoride gets into the crystal of the enamel and turns it into fluorapatite, which takes more acid before it starts to dissolve. It also leaves small deposits of calcium fluoride on the surface, and those deposits are a reservoir: when your mouth turns acid after eating, they release fluoride at the moment it is needed. And while it is still there, it gets in the way of the plaque bacteria when they try to manufacture that acid. All of that depends on how much fluoride is left and how long it stays. What the brush puts in your mouth is not a dose to be swallowed and absorbed, like a syrup. It is a coating that has to stay where it is while you sleep.

When you fill your mouth with water and spit, the concentration of fluoride in your saliva collapses within seconds. The reservoir you had just finished laying down goes down the drain. You did your two minutes, you used up the good paste, and then you mopped a floor you had just painted.

The prevention guide used by dentists in the British public health system, Delivering better oral health, says it in four words: spit, do not rinse. The National Health Service repeats it on its page for patients, along with the advice not to eat or drink anything for the next half hour. The reason is a matter of clock: without rinsing, the fluoride in your saliva stays above normal for an hour or two; with rinsing, that spell comes to nothing.

Besides the fluoride, the tube holds a mild abrasive that drags the plaque off, a detergent that makes foam, and flavour so that you do it again tomorrow. None of that repairs a tooth: the only thing that repairs anything is the fluoride, and it is exactly what goes down the drain.

The mouthwash on your shelf has the same problem, and this is where most people go wrong in perfectly good faith. An adult toothpaste carries 1,450 parts per million of fluoride; a daily fluoride mouthwash carries 225. If you brush and then rinse with it, you are not adding anything up: you are swapping one thousand four hundred and fifty for two hundred and twenty-five. Mouthwash is not bad, it is in the wrong place: its place is another moment of the day, mid-afternoon, when you have not just laid down something better.

The night-time brushing is the one that pays off most, for a measurable reason: while you sleep you produce far less saliva, so whatever you leave in place stays there for hours without being diluted. That is why the rinse before bed is the expensive one.

With children the amount of paste changes, not the rule. Up to the age of three, a smear the size of a grain of rice; from three to six, a pea; and they do not rinse either. The amount is measured because some of it gets swallowed, and too much swallowed fluoride during the years when the teeth are forming leaves white marks on the enamel. Getting a child to spit and not rinse costs a week of insisting and lasts a lifetime.

The habit is hard to break because a rinsed mouth feels clean and an unrinsed mouth is left slightly pasty. That residue which bothers you for three minutes is exactly the product you paid for.

Tomorrow night, brush for the two minutes, spit twice if the taste is too strong for you, and leave the tap off. Do not eat or drink anything for the next half hour and, if you use mouthwash, move it to mid-afternoon. The whole change consists of not turning on a tap.