There have been two times where I distinctly remember brushing my tongue with an unnormal amount of toothpaste (basically covering it full of freshly squeezed toothpaste) and where I felt quite severe brain fog afterwards. Then, in the last couple of weeks, I've decided to decimate my normal fluoride toothpaste usage and change the way I brush my teeth. The point of using fluoride in the first place is to strengthen your tooth enamel, and that's it. The cleaning action of the brush itself is actually sufficient to remove dirt and gunk. So the way I brush my teeth now is I use a tiny amount of toothpaste (maybe half the size of a pea), brush my teeth with it for three quick "rounds" (takes approximately 10 seconds) until my teeth are nicely covered, then I immediately flush my mouth with water while brushing maybe three times, flushing every round. Then, without any toothpaste in my mouth, I clean my tongue with the toothbrush while flushing my mouth with water another three times. The idea is "quick on, quick off", while the brushing does the work of removing the toothpaste while also cleaning the teeth. But why? Among other things, fluoride has been shown to deplete glutamate in the brain, the primary excitatory neurotransmitter, meaning your neurons will be generally less able to fire. Interestingly though, it does this by elevating the activity of the enzyme glutamic acid decarboxylase (GAD), which is how your brain produces GABA from glutamate. So fluoride essentially works as a quite generalized tranquilizer drug. That could actually come in handy if you're having anxiety issues and you only have toothpaste available, although fluoride is toxic in other ways which would probably not make that a good idea. Anyways, the poison is in the dosage. It's probably smart to use some amount of fluoride to avoid tooth decay, but you can also be smarter in the ways you use fluoride (and how much you use) to avoid unnecessary side effects.