Toothbrush, horse hair, your finger, el-cheapy toothpaste, triple stripe, baking soda, bleach, nothing, toxic waste, caterpillar juice, oil or so-on and so forth.
My SigOt was demanding expensive toothpaste yet again ... and me being a cheapskate (I stock up when the good stuff is on special) I commented that the expensive toothpastes were just the standard stuff with baking soda mixed in.
My SigOt took me literally & demanded that I buy (an new box) of Sodium Bicarbonate for cleaning her teeth. After a few minutes laughing, I realised she was serious, and after throwing the Bicarb of Soda in the basket, spent the rest of shopping trip giggling.
So now she dips the toothbrush in the baking soda, puts toothpaste on & then brushes her teeth. It's got a weird salty flavour.
Tehcnically it shouldn't hurt your teeth, cos it's an alkaline, and incidentally bacteria release acid which dissolves your teeth, and your saliva is also alkaline.