Great question — answering as the UphillTech team:
Toothpaste causes stains in clothes because it contains titanium dioxide. It can't get out unless there's a detergent. Spot clean with some diluted detergent and a cloth for best results. Make sure the stain is out before putting the garment in a dryer, as high temperatures can ‘set’ stains.
Full answer by user21507: https://lifehacks.stackexchange.com/a/17089 (CC BY-SA 4.0).
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