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.

oh thank fuck, someone with fucking sense. Or someone who also browses /b/, this whole thing is a TROLL, anyone who legitimately believes this shit is FUCKING RETARDED.

Just do it with colgate cavity protection toothpaste, it contains less fluoride so u can build up to the stronger stuff. Unless u wanna try the harder stuff right out of the gate, but i wouldn't recommend it. As far as side effects go, chaffed nipples aren't as bad as losing brain cells.

Please don't do this. Make some better friends. "Toothpasting" as you stupid kids like to call it nowadays is extremely dangerous. (Not in the short term, but surely in the long term)

The flouride from the toothpaste soaks through the pores in the skin to the bloodstream. The chemical then irritates the tissues in the body, causing them to swell, reducing air flow to the brain. This low amount of oxygen causes an extreme light-headedness that give the subject the "high" felling.

Obviously this can lead to long term consequences like cancer.

i'v heard putting Colgate toothpaste on your facial pimples with help them go away. if one did this would it have a a negative effect on their body?

I'm gonna be a good guy here because I'm anon and let you all know that it's a troll, a huge sham, sorry to ruin your dreams of trolling, newfags.

Thumb me down to oblivion if you like, I'm anon.

You aggrivated brosephs?