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The main ingredient in toothpaste is some kind of flouride salt, like sodium flouride or stannous flouride. Some cheaper brands of toothpaste will settle with olaflur, a hydrophobic organic flouride salt.

In the mouth salivary amylase breaks this down into a harmless organic chemical and free flouride ion. The flouride is what helps kill bacteria.

However, olaflur, by virtue of being a long chain organic molecule, is somewhat lipophilic, meaning it can pass through phospholipid bilayer membranes (ie cell membranes, ie your skin).

Granted it's not terribly lipophilic though, so it needs some help. The nipples, due to their porous structures (how else do you think milk can be secreted from them), allow for much easier passage than other areas of the body.

Since toothpaste also contains a variety of abrasives to help clear bacterial plaques (such as calcium carbonate), placing the paste tightly against the nipples (such as with duct tape) damages the skin during normal movement and soon allows olaflur to pass through.

Now, as I'm sure we all know, the nipples are erogenous zones. When stimulated they become erect. Like penile erection, this requires increased blood flow to the nipple.

Since the nipples are being stimulated by a combination of the duct tape and the toothpaste (further exacerbated by the mint flavorings) this results in greatly increased bloodflow which allows the now-absorbed olaflur to enter the blood where it then dissociates into the organic ion and flouride ion.

At this point flouride ion is transported to the brain where it passes through the blood-brain barrier due to its small size.

Due to its negative charge it is capable of interferring with ion concentrations in the cerebrospinal fluid and alter the firing rates of neurons in the brain. This is what causes the high that is often reported with pasting.