It's a trend now that brushing your teeth with activated charcoal whitens your teeth, is it true?

It's quite true. Floss your teeth, as you would normally 2 times per day and for more deeply stained teeth, brush with activated charcoal 5 days in a row. You will be thrilled with the results.
Perhaps but... This must be a new trend that I personally haven't heard about. In my opinion you can do the same thing with any number of toothpastes and proper brushing and flossing. I certainly don't recommend that you need this new untested and undocumented fad.
Caution... Anything that is too abrasive and used too often can wear away enamel, which in the long run will make your teeth look more yellow. There are no scientific studies to corroborate any claims or to address any potential damage from extended use. Until proven that charcoal actually whitens teeth and does not abrade the enamel, it is difficult to recommend its use.

Related Questions

Will brushing my teeth with activated charcoal harm my composite fillings?

No. Activated charcoal will not harm the filling. For good health - Have a diet rich in fresh vegetables, fruits, whole grains, milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco, alcohol, weed or street drugs in any form. Drink enough water daily so that your urine is mostly colorless. Practice safe sex. Get HPV vaccine. Read more...