the usage of "more than"

Solution 1:

For this question, you guys are looking at the phrase "If more than used for brushing". Instead, look at the whole sentence, "If more than used for brushing is accidentally swallowed, get medical help...".

If you look at the whole phrase, then you can see that the sentence is really saying, "If you accidentally swallow more toothpaste than usually used for brushing, get medical help...".

Solution 2:

MORE is a noun here, meaning " an extra amount or extent" and it is qualified by an adjective phrase " than used for brushing" in reality a reduced noun clause– what's used​ for brushing, object to the preposition ' than '.

The comparison is between ' more ' and ' used for brushing(amount) ' , and together they form a moun phrase subject to the verb ' is swallowed '. To get straight at the meaning we would suggest to segregate " than used for brushing " with commas before and after.