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Government of South Australia and NHS recommend:

After brushing, spit out any excess toothpaste.

Don't rinse your mouth immediately after brushing, as it'll wash away the concentrated fluoride in the remaining toothpaste.

This dilutes it and reduces its preventative effects.

  1. Even after spitting, wouldn't some toothpaste linger in the mouth?

  2. After brushing, it's obviously impractical to keep returning to a sink to spit every time you have saliva. So wouldn't you swallow the residual toothpaste?

  3. If 2 is true, isn't swallowing any amount of toothpaste unhealthy?

  • Who says swallowing any amount of toothpaste is unhealthy? – Carey Gregory Nov 19 '18 at 5:12
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    I didn't say it was. I'm just guessing that small amounts of toothpaste are harmless, as evidenced by the lack of morbidity and mortality attributed to toothpaste. So I'm just questioning why you think swallowing any amount of toothpaste is unhealthy. – Carey Gregory Nov 19 '18 at 5:46
  • @CareyGregory A small amount by accident is probably harmless, but the recommendations overhead would imply a small amount everyday twice? – NNOX Apps Nov 19 '18 at 5:57
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    This question is based on a hypothesis which is not strongly supported. Unless this question is supported with references to support the possibility that the advice given is incorrect this question should be put on hold until it has – Chris Rogers Nov 19 '18 at 9:49
  • @ChrisRogers 'based on a hypothesis which is not strongly supported': Please specify which? – NNOX Apps Nov 20 '18 at 0:16
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Flouridated toothpastes are poisonous but as with anything our good friend Paracelsus comes into play here:

All things are poison, and nothing is without poison, the dosage alone makes it so a thing is not a poison.

By spitting the excess toothpaste out you are simply avoiding ingesting it in sufficient quantities to cause adverse effects.

The advice not to rinse is to allow the fluoride that remains in the mouth to do what it's there for - helping prevent tooth decay, yes you will end up ingesting small amounts of the toothpaste that remains but it's much, much, smaller than the quantity you would have ingested by swallowing the toothpaste rather than spitting it out. As per Crawford et al the lowest dosage of fluoride seen producing symptoms was 1.6 mg/kg (the mean amount in asymptomatic patients was much higher at 3.89 mg/kg) - the classic "pea-sized" lump of toothpaste is ~ 0.25g so taking me as an example..

I weigh 59kg, meaning that using the lower 1.6mg/kg figure I'd have to be ingesting 0.0944g of fluoride before symptoms were even a possibility - looking at a relatively typical fluoridated toothpaste it has 1450ppm of Sodium Fluoride which means our initial 0.25g of toothpaste contains about 0.0003625g of sodium fluoride. Which is 0.145% of that initial 0.25g! So even if all of the initial 0.25g were ingested it doesn't even come close to the levels needed to cause symptoms

So it's not sounding as though that small an amount stands much chance of causing a problem. If we look at the other end of the scale the mean dosage of those who were symptomatic is 4.78 mg/kg so at my weight that would mean ingesting 0.28202g of sodium fluoride, which at 1450 mg/kg would mean eating a frankly insane amount of toothpaste! So I won't lose any sleep over the negligible amounts I may ingest from not rinsing, if I do rinse however I'm going to be removing the opportunity for the fluoride to do it's work on the teeth and removing a known benefit for avoiding an essentially negligible risk isn't a sensible trade-off.

So coming back to your three questions:

Even after spitting, wouldn't some toothpaste linger in the mouth?

Yes, that's the point.

After brushing, it's obviously impractical to keep returning to a sink to spit every time you have saliva. So wouldn't you swallow the residual toothpaste?

A tiny bit yes.

If 2 is true, isn't swallowing any amount of toothpaste unhealthy?

Nope. The dose makes the poison and the dose is insufficient to cause health problems.

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    I weigh 59kg, meaning that using the lower 1.6mg/kg figure I'd have to be ingesting 0.0944g before symptoms were even a possibility - which is 37.76% of that initial 0.25g! The math is wrong, because toothpaste does not consist entirely of Fluor. I would be surprised if more than 10% of the weight is coming from Fluor -> so you‘d need to eat ~1g of toothpaste for any symptoms to show up, at the least. It depends on the mass percentage of fluoride in toothpaste. – Narusan Nov 19 '18 at 13:02
  • @Narusan You're correct of course, that'll teach me for reading material too quickly! – motosubatsu Nov 19 '18 at 13:09
  • You need to update this answer with an edit. And keeping in mind that there aredifferent formulas out there – for converting ppm2vol as well different fluorides – the most typical tp dose was calculated for you here ncbi.nlm.nih.gov/pubmed/3288434 But this back-of-the-envelope answer would be even better with 1.: the actual formulas, 2. another good ref that takes a direct risk assessment. Then differentiate between acute and long term habit. – LаngLаngС Nov 19 '18 at 13:19
  • @LangLangC Yeah it's a bit back-of-the-envelope, I'll spend a bit of time later and dig out some more robust formulae – motosubatsu Nov 19 '18 at 13:34
  • You might want to reword the first sentence. It sounds like fluoridated toothpastes are poisonous but may not be poisonous. It's like saying tap water is poisonous because it contains some fluoride. – Jan Nov 20 '18 at 9:51

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